Add a menu item for triggering WebUI for the Ethereum wallet

[bbondy]

Test Plan

brave/brave-core#2496

This should only be on when the feature flag from #4578 is on.

[LaurenWags]

@bbondy @ryanml since 0.67.x packaged builds are official builds - is it enough to ensure that the menu item specified in test plan (brave/brave-core#2496) is not available and that visiting brave://wallet does not work?

[bbondy]

I think we should just for now make sure that it doesn't show up in the builds. Then when we release the feature we should probably look at all the closed issues that were behind a flag.

[kjozwiak]

Verification PASSSED on macOS 10.14.5 x64 using the following build:

Brave	0.67.117 Chromium: 76.0.3809.62 (Official Build) (64-bit)
Revision	7b77856b3aa34d72f246d12340fc1ded8b2c0e83-refs/branch-heads/3809@{#798}
OS	Mac OS X

• ensured that Crypto Wallet isn't visible under brave://settings
• ensured that Crypto Wallet isn't visible anywhere under the hamburger menu
• ensured that Crypto Wallet isn't visible anywhere under the user profile menu
• ensured that brave://wallet can't be reached/resolved
• ensured that chrome://wallet can't be reached/resolved
• ensured that chrome://wallet isn't listed under brave://about/

Verified passed with macOS 10.13.6 using:

Brave	0.67.117 Chromium: 76.0.3809.62 (Official Build) (64-bit)
Revision	7b77856b3aa34d72f246d12340fc1ded8b2c0e83-refs/branch-heads/3809@{#798}
OS	Mac OS X

• ensured that Crypto Wallet isn't visible under brave://settings
• ensured that Crypto Wallet isn't visible anywhere under the hamburger menu
• ensured that Crypto Wallet isn't visible anywhere under the user profile menu
• ensured that brave://wallet can't be reached/resolved
• ensured that chrome://wallet can't be reached/resolved
• ensured that chrome://wallet isn't listed under brave://about/

Verification passed on

Brave	0.67.117 Chromium: 76.0.3809.62 (Official Build) (64-bit)
Revision	7b77856b3aa34d72f246d12340fc1ded8b2c0e83-refs/branch-heads/3809@{#798}
OS	Ubuntu 18.04 LTS

• ensured that Crypto Wallet isn't visible under brave://settings
• ensured that Crypto Wallet isn't visible anywhere under the hamburger menu
• ensured that Crypto Wallet isn't visible anywhere under the user profile menu
• ensured that brave://wallet can't be reached/resolved
• ensured that chrome://wallet can't be reached/resolved
• ensured that chrome://wallet isn't listed under brave://about/

Verification passed on

Brave	0.67.117 Chromium: 76.0.3809.62 (Official Build) (64-bit)
Revision	7b77856b3aa34d72f246d12340fc1ded8b2c0e83-refs/branch-heads/3809@{#798}
OS	Windows 10 OS Version 1803 (Build 17134.523)

• ensured that Crypto Wallet isn't visible under brave://settings
• ensured that Crypto Wallet isn't visible anywhere under the hamburger menu
• ensured that Crypto Wallet isn't visible anywhere under the user profile menu
• ensured that brave://wallet can't be reached/resolved
• ensured that chrome://wallet can't be reached/resolved
• ensured that chrome://wallet isn't listed under brave://about/

[kjozwiak]

Verification PASSED on macOS 10.14.6 x64 using the following build:

Brave	0.69.129 Chromium: 77.0.3865.90 (Official Build) (64-bit)
Revision	58c425ba843df2918d9d4b409331972646c393dd-refs/branch-heads/3865@{#830}
OS	macOS Version 10.14.6 (Build 18G95)

• ensured that both brave://wallet & chrome://wallet loads the crypto page without issues
• ensured that Crypto Wallets under the hamburger menu loads the crypto page without issues
• ensured that Crypto Wallets under the brave://settings header loads the crypto page without issues
• ensured that chrome://wallet isn't listed under brave://about (same behaviour has `chrome://rewards which also isn't listed)

Verification passed on

Brave	0.69.129 Chromium: 77.0.3865.90 (Official Build) (64-bit)
Revision	58c425ba843df2918d9d4b409331972646c393dd-refs/branch-heads/3865@{#830}
OS	Windows 10 OS Version 1803 (Build 17134.1006)

• ensured that both brave://wallet & chrome://wallet loads the crypto page without issues
• ensured that Crypto Wallets under the hamburger menu loads the crypto page without issues
• ensured that Crypto Wallets under the brave://settings header loads the crypto page without issues
• ensured that chrome://wallet isn't listed under brave://about (same behaviour has `chrome://rewards which also isn't listed)

Verification passed on

Brave	0.69.129 Chromium: 77.0.3865.90 (Official Build) (64-bit)
Revision	58c425ba843df2918d9d4b409331972646c393dd-refs/branch-heads/3865@{#830}
OS	Ubuntu 18.04 LTS

• ensured that both brave://wallet & chrome://wallet loads the crypto page without issues
  

• ensured that Crypto Wallets under the hamburger menu loads the crypto page without issues
  

• ensured that Crypto Wallets under the brave://settings header loads the crypto page without issues

• ensured that chrome://wallet isn't listed under brave://about (same behaviour has `chrome://rewards which also isn't listed)