fix cosmetic filtering

[Snuupy]

Submitter Checklist:

This PR depends on #3002

Fixes cosmetic filtering for tabs loaded in background

Fix brave/brave-browser#5215 (blocking in background tabs)

Fixes stylesheet injection for cosmetic filtering

Fix brave/brave-browser#3041
Fix brave/brave-browser#4348
Fix brave/brave-browser#4338

address 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:

Background tabs

Open http://raymondhill.net/ublock/adbox.html
Right click, enter .ADBox in the field
Bookmark page
Right click bookmark and open as a tab
Switch to new tab and verify that the element is blocked when you switch to it

Cosmetic filtering stylesheet injection

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]

Since this PR was rebased, but #3002 was not, and this PR contains the commit from #3002, and CI passed for this PR, I'm going to close #3002 and merge the commits via this PR, including the issues that both PRs fix in this one.

[petemill]

Works great!!