bugfix(shields): Fix pulse on some subreddits with Shields and Night Mode enabled on iOS

[StephenHeaps]

• In SelectorsPollerScript.js, every 1 second we check if the cosmetic filtering stylesheet is the last element in the document.body. If it is not last, we remove the stylesheet and re-append to document.body so we are the last element (this is so our stylesheet takes priority over previous style elements). However, the DarkReader javascript for Night Mode also does this, which results in a loop of our style element and DarkReader style element competing to be the last element. Every time we move our style element / dark reader moves it's style element, we see the text animate on some old reddit subreddit styles.
  • NOTE: The animation will still occur when the page first loads with Night mode enabled & shields disabled (seems to be an issue with DarkReader). This PR prevents the animation from occurring repeatedly every 1 second when Night mode and Shields are enabled.
• Store the last element we moved below to avoid repeatedly moving the stylesheet to be last.

Resolves brave/brave-browser#43096

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:

1. Visit https://old.reddit.com/r/CalgaryFlames
2. Enable Night Mode & Shields
3. Verify text does not repeatedly animate size / pulse.
   • The pulse happens on initial load, but occurs even with shields disabled (May be subreddit style related).
4. Visit https://old.reddit.com/r/antivirus/comments/1hshe2l (or any comments page on r/antivirus)
5. Verify text does not repeatedly animate size / pulse.
   • The pulse happens on initial load, but occurs even with shields disabled (May be subreddit style related).

Reddit.Night.Mode.+.Shields.animation.fixed.mp4