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 f05fa3f98e36b743b761173552a35f7a85c8defa..851674a4da3b2b27eb3447b504bdb4704069101c 100644
--- a/media/filters/ffmpeg_video_decoder.cc
+++ b/media/filters/ffmpeg_video_decoder.cc
@@ -179,11 +179,12 @@ void FFmpegVideoDecoder::Reset(const base::Closure& closure) {
     return;
   }
 
+  DCHECK(reset_cb_.is_null());
+  reset_cb_ = closure;
+
   if (decryptor_)
     decryptor_->CancelDecrypt();
 
-  reset_cb_ = closure;
-
   // Defer the reset if a read is pending.
   if (!read_cb_.is_null())
     return;
@@ -207,11 +208,12 @@ void FFmpegVideoDecoder::Stop(const base::Closure& closure) {
     return;
   }
 
+  DCHECK(stop_cb_.is_null());
+  stop_cb_ = closure;
+
   if (decryptor_)
     decryptor_->CancelDecrypt();
 
-  stop_cb_ = closure;
-
   // Defer stopping if a read is pending.
   if (!read_cb_.is_null())
     return;