Fix custom ad notification z-ordering and possible display issues.

[aseren]

Resolves brave/brave-browser#22203

Submitter Checklist:

• I confirm that no security/privacy review is needed, or that I have requested one
• 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 lint, 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/Custom-Headers
  • 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:

The issue should be tested on Windows and MacOS platforms.

• Clean install
• Run Brave with: --enable-features="CustomAdNotifications:should_attach_ad_notification_to_browser_window/true"
• Enable rewards and ads
• Trigger a notification ad
• Check that the notification ad has the same Z-order as the Browser window

UPD: Features names in Brave Nightly were refactored. Nightly parameter is renamed to use_same_z_order_as_browser_window and it is turned on by default. So for Nightly now it is needed to enable CustomNotificationAds feature only with --enable-features=CustomNotificationAds or using a Flag in brave://flags

[tmancey]

nit: . at end of sentence

[aseren]

All other comments in this file don't have . at the end of a sentence. Maybe it is better to fix all of them as a followup?

[tmancey]

Sounds good to me

[tmancey]

I will raise a PR over the weekend as in the process of adding some public facing comments anyhow. Thanks

[tmancey]

LGTM++