Adding in ffmpeg_common.h into header for definition of CodecID.

media/base/mock_ffmpeg.h

Index: media/base/mock_ffmpeg.h
diff --git a/media/base/mock_ffmpeg.h b/media/base/mock_ffmpeg.h
index 62b085c0bb7bff304d3effa1be82970db4f2144f..711cf1520605344510fd50a890469e79a56334f7 100644
--- a/media/base/mock_ffmpeg.h
+++ b/media/base/mock_ffmpeg.h
@@ -5,13 +5,10 @@
 #ifndef MEDIA_BASE_MOCK_FFMPEG_H_
 #define MEDIA_BASE_MOCK_FFMPEG_H_
 
-#include "testing/gmock/include/gmock/gmock.h"
+// TODO(scherkus): See if we can remove ffmpeg_common from this file.
+#include "media/filters/ffmpeg_common.h"
 
-struct AVCodec;
-struct AVCodecContext;
-struct AVFrame;
-struct AVPacket;
-enum CodecID;
+#include "testing/gmock/include/gmock/gmock.h"
 
 namespace media {