stylesheet injection for cosmetic filtering

[Snuupy]

Submitter Checklist:

closes brave/brave-browser#3041, brave/brave-browser#4348, brave/brave-browser#4338

and brave/brave-browser#94 (comment)

• 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:

Block element with !important html tag or is asynchronously added to the webpage (ex. reddit/facebook with filters added) and see that those ads are blocked.

• 1. Visit http://raymondhill.net/ublock/adbox.html (mirror: https://jsfiddle.net/fbdagveq/)
• 2. Right click, add .ADBox filter
• 3. master will not be able to block the 2nd red highlighted element (because of !important), PR will.

• 1. Visit https://new.reddit.com/
• 2. add .promotedlink to filter
• 3. scroll down to see promoted links/advertisements (master will not be able to block the promoted links, but this PR will)

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.

[petemill]

@Snuupy I'm sure this makes sense, but please can you put a test plan for each issue (on the issues preferrably) that is not reproducible on this branch, but is reproducible in master.

For example I tried the STR in brave/brave-browser#4338 but could not see a difference between master and this branch. I added a custom rule in both versions for #header-right-section and it seemed to work for both.

Thanks!

[Snuupy]

@Snuupy I'm sure this makes sense, but please can you put a test plan for each issue (on the issues preferrably) that is not reproducible on this branch, but is reproducible in master.

For example I tried the STR in brave/brave-browser#4338 but could not see a difference between master and this branch. I added a custom rule in both versions for #header-right-section and it seemed to work for both.

Thanks!

@petemill

I have 2 tests that show this PR works (and the master branch does not):

• 1. Visit http://raymondhill.net/ublock/adbox.html (mirror: https://jsfiddle.net/fbdagveq/)
• 2. Right click, add .ADBox filter
• 3. master will not be able to block the 2nd red highlighted element (because of !important), PR will.

• 1. Visit https://new.reddit.com/
• 2. add .promotedlink to filter
• 3. scroll down to see promoted links/advertisements (master will not be able to block the promoted links if they are loaded after the page has finished loading, but this PR will)

Edited PR text to include the above instructions.

[petemill]

Closed in favor of #3121