Disable cosmetic filtering on Speedreader pages

[boocmp]

Resolves brave/brave-browser#40642

Submitter Checklist:

• I confirm that no security/privacy review is needed and no other type of reviews are needed, or that I have requested them
• There is a ticket for my issue
• Used Github auto-closing keywords in the PR description above
• Wrote a good PR/commit description
• Squashed any review feedback or "fixup" commits before merge, so that history is a record of what happened in the repo, not your PR
• 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 wiki
  • npm run presubmit wiki, 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/Web-Compatibility-Exceptions-in-Brave
  • https://github.com/brave/brave-browser/wiki/QA-Guide
  • https://github.com/brave/brave-browser/wiki/P3A

Test Plan:

[iefremov]

if I understand correctly, we only disable CF, not all shields on distilled pages, right? So this is something different from what the task requires. So we'd better to either modify task and rename PR ("Disable cosmetic filtering on speedreader pages") or properly disable shields

[boocmp]

@ShivanKaul Yes, you are right. The actual disabling of shields for the specified tab will require quite a lot of changes, because now the settings work at the ContentSettings level. In general, this could be useful, but so far there has been no need to disable shields for a specific tab (AFAIK). Disabling CF is enough to solve problems with the reading mode I would even prefer not to disable the shields button.

[ShivanKaul]

@boocmp @iefremov We do have issues with Speedreader + network blocking too: https://bravesoftware.slack.com/archives/CLX7EV42Z/p1726603890011589?thread_ts=1726595972.638809&cid=CLX7EV42Z

[ShivanKaul]

Spoke with @boocmp and I understand now that network blocking is not relevant here, at least for the issue I mentioned. Let's go with just CF disabling.