Fix test flakiness in UserPromptingChecker

[emaxx-google]

Fix two errors flakily observed in unit tests:

• "Cannot read properties of undefined" when trying to call into chrome.app.window.get().
  • Solution: we shouldn't call this function in unit tests, hence stub it out.
• "Unhandled promise rejection" inside PolicyOrPromptingPermissionsChecker.
  • Solution: suppress these errors since they aren't useful. This happens in the checkByUserPromptingChecker_() method where we create a child promise that's intentionally not returned anywhere.

[github-actions[bot]]

Continuous Integration: All tests passed. C/C++ test coverage: 73.34% lines.

[github-actions[bot]]

Continuous Integration: All tests passed. C/C++ test coverage: 73.36% lines.