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 ff44bf8a70c6cca00dc561e181d98736767bcabe..3ad262f55250e4ae2b59c73a11f1ceada8cea692 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;
@@ -207,16 +208,18 @@ void FFmpegVideoDecoder::Stop(const base::Closure& closure) {
     return;
   }
 
+  DCHECK(stop_cb_.is_null());
+
   if (state_ == kUninitialized) {
     closure.Run();
     return;
   }
 
+  stop_cb_ = closure;
+
   if (decryptor_)
     decryptor_->CancelDecrypt();
 
-  stop_cb_ = closure;
-
   // Defer stopping if a read is pending.
   if (!read_cb_.is_null())
     return;