Rewrite getUseMedia calls to use promises.

This removes many uses of waitForVideo() etc. and makes the
control flow of the test clearer. Instead of having the
all-events-occurred handler magically calling reportTestSuccess,
it's laid out explicitly in each test.

Bug: chromium:777857
Change-Id: I034d8950f764df607069b3e4b5ce6baeb46eab9b
Reviewed-on: https://chromium-review.googlesource.com/758848
Commit-Queue: Patrik Höglund <phoglund@chromium.org>
Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Cr-Commit-Position: refs/heads/master@{#515917}