[ubsan] Fix broken TestingPrefServiceSimple downcasts

[cdesouza-chromium]

The return type for this function has changed for a while now, so it appears we have been running over undefined behaviour all along.

This change corrects the downcast to match the type returned by GetPrefs(), namely, SearchEnginesTestEnvironment, which in turn has sync_preferences::TestingPrefServiceSyncable for its own prefs_service() method.

Chromium changes:
https://chromium.googlesource.com/chromium/src/+/2862d7f6b0842b53209c8038ec54eba132b55d8b https://chromium.googlesource.com/chromium/src/+/28881de3ad0bc41236fb00a794939ec7c0bff819

commit 2862d7f6b0842b53209c8038ec54eba132b55d8b
Author: Jack Yammine <[email protected]>
Date:   Thu Jul 18 11:20:17 2024 +0000

    Remove deprecated TemplateURLService constructor used in tests

    This is part of a multi-CL change [2/4].

    We remove the TemplateURLService constructor that was only used for
    tests and update the tests accordingly.

    Bug: 40287734

commit 28881de3ad0bc41236fb00a794939ec7c0bff819
Author: Jack Yammine <[email protected]>
Date:   Fri Jul 12 12:11:33 2024 +0000

    Correctly initialize TemplateURLService in tests

    We currently pass nullptr for PrefService and SearchEngineChoiceService
    in tests. This leads to having code paths in production that need
    to handle theses cases.

    This is part of a multi-CL change.

    Bug: 40287734

Resolves brave/brave-browser#41435

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:

[brave-builds]

Released in v1.72.66