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Currently these tests only provide audio_time_to_play
and video_time_to_play media metrics (along with generic
cpu and memory metrics). Adding new metrics after these
tests are committed simply means adding trace events to
the product code and turning those into metrics in
media_metric.html. In this way, the code I have added
is just the scenarios.

These scenarios are meant to be working examples
that can be edited significantly in the future.
That said, once this is committed, we're probably
fine dropping the legacy mse_cases (since they are
really flaky and haven't been finding any bugs.)
These tests will be enough to cover them.

Bug: 772971,713335
Change-Id: Ib2c32e4ab97738cc8d8e4b745b11820761213ad3
Reviewed-on: https://chromium-review.googlesource.com/733985
Commit-Queue: Caleb Rouleau <crouleau@chromium.org>
Reviewed-by: John Chen <johnchen@chromium.org>
Reviewed-by: Matthew Wolenetz <wolenetz@chromium.org>
Reviewed-by: Ned Nguyen <nednguyen@google.com>
Cr-Commit-Position: refs/heads/master@{#513002}