Listen for referral headers updates

[emerick]

Fixes brave/brave-browser#2020

We were previously only retrieving the referral headers on first
launch. Aside from preventing us from incorporating potential updates
to the headers, this also made it so that a browser restart was
required the first time a user initialized the referral (as the
referral headers would still be empty from the network delegate's
perspective).

Now we register for changes to that preference and pull them in on the
fly.

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
  • Windows
  • macOS
  • Linux
• Verified that these changes pass automated tests (npm test brave_unit_tests && npm test brave_browser_tests) on
  • Windows
  • macOS
  • Linux
• 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

Test Plan:

• Delete existing profile if any
• Download referral version of Brave (e.g., BraveBrowserSetup-dowjones-marketwatch.exe)
• Install referral version of Brave
• Wait for promo tab to load
• Sign up for promo and visit a URL belonging to the site
• Ensure that headers include appropriate promo-specific headers, e.g. X-Brave-Partner

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

[emerick]

master: c532659
0.58.x: 3871c13
0.57.x: b9fee322

[kjozwiak]

@emerick can we get this uplifted into dev so it can make it into 0.58.x when it moves into beta next week? There's no approval requirement to land things into dev at this moment.

[bbondy]

does @aekeus need this for 0.57.x?

[emerick]

@aekeus @kjozwiak Uplifted to dev. Please let me know if I should request uplift to 0.57.x as well.

[srirambv]

@emerick uplift approved for 0.57.x. Please update label once merged to 0.57.x

[bbondy]

0.57.x has wrong changeset above but it is actually landed:
669baa8