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Currently cast_media_unittests can't build on platforms
(enable_video_with_mixed_audio == true && use_alsa == false), because of
not finding the VideoDecoderForMixer::InitializeGraphicsForTesting
symbol.

This symbol isn't useful on those platforms, so define a dummy that can be used in
this case.

BUG=b/124520050
TEST=Build cast_media_unittests

Change-Id: I35abfc3a549cd1fa185534d7b48d8dd93db14e04
Reviewed-on: https://chromium-review.googlesource.com/c/1475048
Commit-Queue: Mina Almasry <almasrymina@chromium.org>
Reviewed-by: Kenneth MacKay <kmackay@chromium.org>
Reviewed-by: Luke Halliwell <halliwell@chromium.org>
Auto-Submit: Mina Almasry <almasrymina@chromium.org>
Cr-Commit-Position: refs/heads/master@{#633876}