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This test base instantiates a content::ChildProcess to provide the
expected environment for the code-under-test. ChildProcess expects to
either run in the context of ScopedTaskEnvironment, in tests, or to
need to initialize the TaskScheduler itself.

Add a ScopedTaskEnvironment to the test base, otherwise the ChildProcess
TaskScheduler will "leak" and break later tests.

Re-enables the RTCVideoDecoderAdapterTest tests, that had been
broken by this issue.

TBR=hbos

Bug: 875278, 875301, 875336
Change-Id: I8a7ee1f42e62241d48720c41897bdcadbcd26bdf
Reviewed-on: https://chromium-review.googlesource.com/1180408
Commit-Queue: Wez <wez@chromium.org>
Reviewed-by: Wez <wez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#584298}