fixes ads on yt and gets rid of csp error

[SergeyZhukovsky]

Resolves brave/brave-browser#14359

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:

described in the original issue

[antonok-edm]

I can't speak for the isolated world stuff, but the scriptlet injection method looks good to me 👍

[SergeyZhukovsky]

post-init failed due to audit problem which is not related to the PR

[stephendonner]

Looks good using:

Brave	1.23.5 Chromium: 89.0.4389.58 (Official Build) nightly (x86_64)
Revision	1a139f28ecc27719439e37c6b1533cee999cb802-refs/branch-heads/4389@{#1134}
OS	macOS Version 11.2.2 (Build 20D80)

Steps from brave/brave-browser#14359

1. Sign in to YouTube using non-Premium YouTube/Google account
2. Open a video link https://www.youtube.com/watch?v=xXMW9VNMPT4
3. Make sure it plays and pick some other video from a right panel
4. Make sure second video plays, click Back arrow on a browser panel to come back to the first video

Verified that upon clicking Back I didn't get any ad or ad overlay:

I did notice that going back and forward a few times triggers the ad overlay, but in talking with @SergeyZhukovsky it sounds like a separate issue to be investigated/clarified further, and not part of the original issue here (which only concerns an initial Back action).

Confirmed parity with

Brave	1.20.110 Chromium: 88.0.4324.192 (Official Build) (x86_64)
Revision	31b458a18f133db9203eb5a5dd6552de0716dda3-refs/branch-heads/4324_182@{#6}