Add video decoding methods in Decryptor and add DecryptingVideoDecoder.

media/filters/ffmpeg_video_decoder.cc

Index: media/filters/ffmpeg_video_decoder.cc
diff --git a/media/filters/ffmpeg_video_decoder.cc b/media/filters/ffmpeg_video_decoder.cc
index 097ebb01cff6858376757d9c539ae1deab444c49..0c9637fe7f2f1f530bbd7e36984e959edbea7e18 100644
--- a/media/filters/ffmpeg_video_decoder.cc
+++ b/media/filters/ffmpeg_video_decoder.cc
@@ -179,6 +179,7 @@ void FFmpegVideoDecoder::Reset(const base::Closure& closure) {
     return;
   }
 
+  DCHECK(reset_cb_.is_null());
   reset_cb_ = closure;
 
   // Defer the reset if a read is pending.
@@ -204,11 +205,12 @@ void FFmpegVideoDecoder::Stop(const base::Closure& closure) {
     return;
   }
 
+  DCHECK(stop_cb_.is_null());
+  stop_cb_ = closure;
+
   if (decryptor_)
     decryptor_->Stop();
 
-  stop_cb_ = closure;
-
   // Defer stopping if a read is pending.
   if (!read_cb_.is_null())
     return;