Fix VideoFrameGenerator so Flush() callback always runs after a pending Read() callback.

media/filters/video_frame_generator.cc

Index: media/filters/video_frame_generator.cc
diff --git a/media/filters/video_frame_generator.cc b/media/filters/video_frame_generator.cc
index 3141399c369924b3f35773ebd998c1b631bc33d3..38dbdecc0076593588bbe5980d65f7271175977b 100644
--- a/media/filters/video_frame_generator.cc
+++ b/media/filters/video_frame_generator.cc
@@ -39,6 +39,10 @@ void VideoFrameGenerator::Read(const ReadCB& read_cb) {
       base::Bind(&VideoFrameGenerator::ReadOnDecoderThread, this, read_cb));
 }
 
+void VideoFrameGenerator::Flush(const base::Closure& flush_cb) {
+  message_loop_proxy_->PostTask(FROM_HERE, flush_cb);
+}
+
 const gfx::Size& VideoFrameGenerator::natural_size() {
   return natural_size_;
 }