Fix a deadlock in the GPU Video decoding code in the renderer which would occur when the renderer

Fix a deadlock in the GPU Video decoding code in the renderer which would occur when the renderer
thread was waiting to get the next frame for renderering which attempts to grab the VideoRendererBase lock
and the gpu video decoder thread would wait on the renderer thread to execute a task while holding the above
lock.

Fix is to post task the demuxer_stream_->Read call to ensure that the lock is released by the gpu video decoder
thread.

BUG=129010
R=fischman
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=138120

[Ami GONE FROM CHROMIUM, 2012-05-21 18:26:53]

LGTM assuming it fixes the deadlock :)