Only specify net::HostResolverSource::DNS when DoH is enabled

[darkdh]

Resolves brave/brave-browser#14755

When DoH setting is automatic or off, it should be using system resolver other than forcing it to use async resolver

Submitter Checklist:

• I confirm that no security/privacy review is needed, or that I have requested one
• There is a ticket for my issue
• Used Github auto-closing keywords in the PR description above
• Wrote a good PR/commit description
• Added appropriate labels (QA/Yes or QA/No; release-notes/include or release-notes/exclude; OS/...) to the associated issue
• Checked the PR locally: npm run test -- brave_browser_tests, npm run test -- brave_unit_tests, npm run lint, npm run gn_check, npm run tslint
• Ran git rebase master (if needed)

Reviewer Checklist:

• A security review is not needed, or a link to one is included in the PR description
• New files have MPL-2.0 license header
• Adequate test coverage exists to prevent regressions
• Major classes, functions and non-trivial code blocks are well-commented
• Changes in component dependencies are properly reflected in gn
• Code follows the style guide
• Test plan is specified in PR before merging

After-merge Checklist:

• The associated issue milestone is set to the smallest version that the
  changes has landed on
• All relevant documentation has been updated, for instance:
  • https://github.com/brave/brave-browser/wiki/Deviations-from-Chromium-(features-we-disable-or-remove)
  • https://github.com/brave/brave-browser/wiki/Proxy-redirected-URLs
  • https://github.com/brave/brave-browser/wiki/Fingerprinting-Protections
  • https://github.com/brave/brave-browser/wiki/Brave%E2%80%99s-Use-of-Referral-Codes
  • https://github.com/brave/brave-browser/wiki/Custom-Headers
  • https://github.com/brave/brave-browser/wiki/Web-Compatibility-Exceptions-in-Brave
  • https://github.com/brave/brave-browser/wiki/QA-Guide
  • https://github.com/brave/brave-browser/wiki/P3A

Test Plan:

No DoH leak

1. Launch Brave and go to brave://settings/security
2. Change DOH setting to secure not automatic by
3. Open wireshark and use dns filter and start recording
4. Open a tab and navigate to https://tools.ietf.org/
5. There shouldn't be any UDP dns query of tools.ietf.org in wireshark

CNAME adblock

test bellowed scenarios with

• DoH off

• DoH automatic

• DoH on

1. visit https://pushbullet.com (there should be 1 request being blocked
2. add the following custom filer via brave://adblock

||stripecdn.map.fastly.net^$important
||d2rhbqunra2aep.cloudfront.net^$important

3. once the above custom filter has been applied, visit https://pushbullet.com (there should be 2 requests blocked)

[bridiver]

lint nit: 4 spaces

[darkdh]

fixed

[darkdh]

audit deps failure is not related to this PR
https://ci.brave.com/job/pr-brave-browser-cname-adblock-doh-post-init/3/execution/node/145/log/

[kjozwiak]

Verification PASSED on Win 10 x64 using the following build:

Brave | 1.24.6 Chromium: 89.0.4389.90 (Official Build) nightly (64-bit)
-- | --
Revision | 62eb262cdaae9ef819aadd778193781455ec7a49-refs/branch-heads/4389@{#1534}
OS | Windows 10 OS Version 2009 (Build 19042.868)

No DoH leak

Prerequisite: Followed https://wiki.wireshark.org/TLS and setup environment

Using the STR mentioned via #8279 (comment), ensured that there wasn't any DNS queries when visiting https://tools.ietf.org:

CNAME adblock

DoH disabled

Disabled	Without filters	Filters applied

DoH automatic

Automatic	Without filters	Filters applied

CloufFlare

Cloudflare	Without filters	Filters applied

OpenDNS

OpenDNS	Without filters	Filters applied