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I suspect that a test wasn't timing out due to slowness, but in fact
because a SINGLETON navigation was switching instead, and not waking up
waiting observers. This CL adds an explicit 'wait' parameter for
choosing (instead of relying on disposition).

It also splits up the respective test into SINGLETON and SWITCH_TO_TAB
versions (they're orthogonal) and cleans up some comments.

Bug: 822071
Change-Id: I241478b7a566b0e843f92b091d8f090ffbf9011c
Reviewed-on: https://chromium-review.googlesource.com/964386
Commit-Queue: Kevin Bailey <krb@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543695}