Enable DNS-over-HTTPS upgrades when possible

[fmarier]

Fixes brave/brave-browser#1864.

This will not change a user's DNS provider. Instead, we will only upgrade to DoH if the OS resolver is known to support DoH (e.g. Cloudflare or Google).

This essentially enables for all users the DoH experiment that Chrome rolled out to 1% of its users: https://docs.google.com/document/d/15Ss0OaJeb-T3g2RMwgikHvsC0CPKd-MLeGeetv1wYY4/edit#

Security review: https://github.com/brave/security/issues/57

Submitter Checklist:

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Verified that these changes build without errors on
  • Android
  • iOS
  • Linux
  • macOS
  • Windows
• Verified that these changes pass automated tests (unit, browser, security tests) on
  • iOS
  • Linux
  • macOS
  • Windows
• Verified that all lint errors/warnings are resolved (npm run lint)
• Ran git rebase master (if needed).
• Ran git rebase -i to squash commits (if needed).
• Tagged reviewers and labelled the pull request as needed.
• Request a security/privacy review as needed.
• Add appropriate QA labels (QA/Yes or QA/No) to include the closed issue in milestone
• Public documentation has been updated as necessary. 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-Protection-Mode
  • 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-issues-with-tracking-protection
  • https://github.com/brave/brave-browser/wiki/QA-Guide

Test Plan:

On Windows configure the browser:

1. Click on the Start menu, then click on Control Panel.
2. Click on Network and Internet.
3. Click on Change Adapter Settings.
4. Right click on the network adapter you are using.
5. Select Internet Protocol Version 4
6. Click Properties.
7. Click Use The Following DNS Server Addresses.
8. Replace those addresses with the 1.1.1.1 DNS addresses: 1.1.1.1 and 1.0.0.1
9. Save the settings.
10. Right-click on the adapter and then click "Disable".
11. Right-click on the adapter and then click "Enable".
12. Start Brave and visit https://1.1.1.1/help.

You should see:

• Connected to 1.1.1.1: Yes
• Using DNS over HTTPS (DoH): Yes

On macOS configure the browser:

You should see:

• Connected to 1.1.1.1: Yes
• Using DNS over HTTPS (DoH): Yes

Reviewer Checklist:

• New files have MPL-2.0 license header.
• Request a security/privacy review as needed.
• Adequate test coverage exists to prevent regressions
• Verify test plan is specified in PR before merging to source

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.

[bsclifton]

Updated milestone to be 1.6 (I think master was 1.5 when this was opened) and also added milestone to brave/brave-browser#1864
cc: @fmarier