bugfix(shields): Remove Image.prototype.src getter/setter readonly protections on iOS

[StephenHeaps]

• The $() wrapper on our Image.prototype.src getter/setter in TrackingProtectionStats.js is causing an error that results in a blank screen on dillards.com and some other sites: [Error] TypeError: Attempted to assign to readonly property.

• TrackingProtectionStats.js is used for us to determine the number of ads/trackers blocked on a page. The potential consequence of removing this protection is the site could manipulate our Image.prototype.src override and we may not know if an image was blocked to count in our tracking stats, however the image would still be blocked (it just won't be included in our "Ads and other creepy things blocked" stats / count). This count is considered an estimate; it's better we don't have webcompat issues than a perfectly correct stats count.

• This same error is also observed on petsafe.com, however there is another issue on that site causing a white screen that is not fixed by this PR.

  • Slimlist is blocking typekit.net css which the page requires to load: [Error] Error: Loading CSS chunk 1869 failed.
  • To verify this, you must comment out / disable the content blocker then visit petsafe.com.
  • petsafe.com will only display with content blocker (or slimlist disabled) AND this PR's fix to remove $() wrapper on Image.prototype.src.

Resolves brave/brave-browser#43286

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://www.dillards.com/ with Shields up
2. Verify webpage is displayed correctly