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A reland of https://crrev.com/c/1324391

This is necessary because we want content::TestBrowserThreadBundle to
own a BrowserUIThreadScheduler, but that also owns a ScopedTaskEnvironment
and you can't have two SequenceManagers on the same thread.

This patch allows ScopedTaskEnvironment to optionally work with an
externally owned SequenceManager solving the problem.

This implements https://docs.google.com/document/d/1y08C6JQ9Yta3EQXzwIqqIIKHq9500WV6CWFZzZfDx7I/edit?usp=drivesdk,

We now have the ability to mock time on the UI and IO threads.

RE HttpServiceTest change HttpServiceTest.MultipleRequests was failing
mysteriously on fuschia on the bots only (not locally). IO_MOCK_TIME seems
to have fixed this.


TBR=gab@chromium.org,fdoray@chromium.org,wez@chromium.org

Bug: 863341, 891670, 708584
Change-Id: I95444a0a50f8b577c5fae62c12c8423c7e8f21d6
Reviewed-on: https://chromium-review.googlesource.com/c/1361863
Commit-Queue: Alex Clarke <alexclarke@chromium.org>
Reviewed-by: Alex Clarke <alexclarke@chromium.org>
Reviewed-by: Sami Kyöstilä <skyostil@chromium.org>
Cr-Commit-Position: refs/heads/master@{#615496}