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These tests were originally made to cover http://crbug.com/398157.
Alas, that bug turned out to be very difficult to test since it does
not affect energy levels. That's what we use as a lightweight way to
detect whether audio is playing, but the bug managed to make audio stop
playing without affecting the levels. Still, I think the tests are
useful.

BUG=402860
R=perkj@chromium.org

Review URL: https://codereview.chromium.org/468243003

Cr-Commit-Position: refs/heads/master@{#291692}