UserScriptListenerTest.NavigationWaitsForContentScriptsToLoad is flaky.

Findit has detected 3 flake occurrences of this test within the
past 24 hours. List of all flake occurrences can be found at:
https://findit-for-me.appspot.com/flake/occurrences?key=ag9zfmZpbmRpdC1mb3ItbWVyXwsSBUZsYWtlIlRjaHJvbWl1bUBicm93c2VyX3Rlc3RzQFVzZXJTY3JpcHRMaXN0ZW5lclRlc3QuTmF2aWdhdGlvbldhaXRzRm9yQ29udGVudFNjcmlwdHNUb0xvYWQM.

Unless the culprit CL is found and reverted, please disable this test first
within 30 minutes then find an appropriate owner.

If the result above is wrong, please file a bug using this link:
https://bugs.chromium.org/p/chromium/issues/entry?status=Unconfirmed&labels=Pri-1,Test-Findit-Wrong&components=Tools%3ETest%3EFindit%3EFlakiness&summary=%5BFindit%5D%20Flake%20Detection%20-%20Wrong%20result%20for%20UserScriptListenerTest.NavigationWaitsForContentScriptsToLoad&comment=Link%20to%20flake%20occurrences%3A%20https://findit-for-me.appspot.com/flake/occurrences?key=ag9zfmZpbmRpdC1mb3ItbWVyXwsSBUZsYWtlIlRjaHJvbWl1bUBicm93c2VyX3Rlc3RzQFVzZXJTY3JpcHRMaXN0ZW5lclRlc3QuTmF2aWdhdGlvbldhaXRzRm9yQ29udGVudFNjcmlwdHNUb0xvYWQM

Automatically posted by the findit-for-me app (https://goo.gl/Ot9f7N).