media::FFmpegVideoDecoder: remove ctor param |task_runner| and use a ThreadChecker instead.

media/filters/ffmpeg_video_decoder.h

Index: media/filters/ffmpeg_video_decoder.h
diff --git a/media/filters/ffmpeg_video_decoder.h b/media/filters/ffmpeg_video_decoder.h
index 8477ad592ab790c6e6f27ab4ea2efc3761b9292f..54a62bebea38f0d14e9f57f26832df527d3fbe58 100644
--- a/media/filters/ffmpeg_video_decoder.h
+++ b/media/filters/ffmpeg_video_decoder.h
@@ -9,6 +9,7 @@
 
 #include "base/callback.h"
 #include "base/memory/scoped_ptr.h"
+#include "base/threading/thread_checker.h"
 #include "media/base/video_decoder.h"
 #include "media/base/video_decoder_config.h"
 #include "media/base/video_frame_pool.h"
@@ -29,8 +30,7 @@ class MEDIA_EXPORT FFmpegVideoDecoder : public VideoDecoder {
  public:
   static bool IsCodecSupported(VideoCodec codec);
 
-  explicit FFmpegVideoDecoder(
-      const scoped_refptr<base::SingleThreadTaskRunner>& task_runner);
+  FFmpegVideoDecoder();
   ~FFmpegVideoDecoder() override;
 
   // Allow decoding of individual NALU. Entire frames are required by default.
@@ -75,7 +75,7 @@ class MEDIA_EXPORT FFmpegVideoDecoder : public VideoDecoder {
   // and resets them to NULL.
   void ReleaseFFmpegResources();
 
-  scoped_refptr<base::SingleThreadTaskRunner> task_runner_;
+  base::ThreadChecker thread_checker_;
 
   DecoderState state_;