The FakeVideoDecoder simulates a typical video decoder that could have some
decoding delays (to simulate out-of-order decoding). Since it's fake,
it does not care about what the input is. It always generate output frames
successfully but with a frame delay (specified in the ctor).

All callbacks passed to FakeVideoDecoder can be held if desired. This gives the
test class a chance to do some operation during pending callbacks.

BUG=141788

Review URL: https://chromiumcodereview.appspot.com/15085011

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@202789 0039d316-1c4b-4281-b951-d872f2087c98