Add support for encrypted WebM files as defined in the RFC.

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 3c4d39a4a7ee90639c189d2d85be5a4ee4b9d09f..92a448883484724e2b65df2692ffbee6061ece41 100644
--- a/media/filters/ffmpeg_video_decoder.h
+++ b/media/filters/ffmpeg_video_decoder.h
@@ -36,7 +36,7 @@ class MEDIA_EXPORT FFmpegVideoDecoder : public VideoDecoder {
 
   // Must be called prior to initialization if decrypted buffers will be
   // encountered.
-  void set_decryptor(AesDecryptor* decryptor);
+  void set_decryptor(HmacAesDecryptor* decryptor);
 
  protected:
   virtual ~FFmpegVideoDecoder();
@@ -102,7 +102,7 @@ class MEDIA_EXPORT FFmpegVideoDecoder : public VideoDecoder {
   // Pointer to the demuxer stream that will feed us compressed buffers.
   scoped_refptr<DemuxerStream> demuxer_stream_;
 
-  AesDecryptor* decryptor_;
+  HmacAesDecryptor* decryptor_;
 
   DISALLOW_COPY_AND_ASSIGN(FFmpegVideoDecoder);
 };