Rewrite FFmpegVideoDecoder tests based on existing FFmpegVideoDecodeEngine tests.
Rewrite FFmpegVideoDecoder tests based on existing FFmpegVideoDecodeEngine tests.

The old tests were mostly useless as they tested using a mocked VideoDecodeEngine.

Review URL: http://codereview.chromium.org/8340008

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@107681 0039d316-1c4b-4281-b951-d872f2087c98

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