Brave Ads intermittently crashes when opening a new tab

[tmancey]

Resolves brave/brave-browser#9393

Submitter Checklist:

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Verified that these changes build without errors on
  • Android
  • iOS
  • Linux
  • macOS
  • Windows
• Verified that these changes pass automated tests (unit, browser, security tests) on
  • iOS
  • Linux
  • macOS
  • Windows
• Verified that all lint errors/warnings are resolved (npm run lint)
• Ran git rebase master (if needed).
• Ran git rebase -i to squash commits (if needed).
• Tagged reviewers and labelled the pull request as needed.
• Request a security/privacy review as needed.
• Add appropriate QA labels (QA/Yes or QA/No) to include the closed issue in milestone
• Public documentation has been updated as necessary. 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-issues-with-tracking-protection
  • https://github.com/brave/brave-browser/wiki/QA-Guide

Test Plan:

• Confirm ads does not crash if tapping a notification while the browser is shutting down (race condition so may need many tries)

Reviewer Checklist:

• New files have MPL-2.0 license header.
• Request a security/privacy review as needed.
• Adequate test coverage exists to prevent regressions
• Verify test plan is specified in PR before merging to source

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.

[emerick]

LGTM

[tmancey]

CI failed due to unrelated gn check and rewards browser tests

[kjozwiak]

Verification PASSED on macOS 10.15.6 x64 using the following build:

Brave | 1.14.21 Chromium: 84.0.4147.105 (Official Build) nightly (64-bit)
-- | --
Revision | a6b12dfad6663f13a7e16e9a42a6a4975374096b-refs/branch-heads/4147@{#943}
OS | macOS Version 10.15.6 (Build 19G73)

Ran through the case outlined via #6304 (comment) about ~x15 times and didn't see any crashes being added into brave://crashes. Went through the following:

• launched 1.14.21 CR: 84.0.4147.105 and enabled rewards
• visited brave.com and triggered an ad

Once triggered, close Brave via Command + Q and ensured the browser didn't crash by checking brave://crashes. Closed the browser in various stages after clicking an ad notification.