Enable prefetch-privacy-changes flag by default

[fmarier]

Resolves brave/brave-browser#8319

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-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-issues-with-tracking-protection
  • https://github.com/brave/brave-browser/wiki/QA-Guide

Test Plan:

Open https://fmarier.github.io/brave-testing/prefetch.html and follow the instructions.

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.

[simonhong]

Changes looks good.
Can you add this feature to BraveMainDelegateBrowserTest.EnabledFeatures test case?

[fmarier]

Can you add this feature to BraveMainDelegateBrowserTest.EnabledFeatures test case?

Thanks @simonhong, I wasn't aware of that test.

[fmarier]

The only CI step which failed was https://ci.brave.com/job/pr-brave-browser-francois-enable-prefetch-privacy-changes-8319/3/execution/node/621/log/ and the log doesn't contain any errors.

Merging as discussed in https://bravesoftware.slack.com/archives/CA5FPHWLF/p1591303866087700.