Add a histogram for video decoder fallback.

media/filters/decoder_stream.h

Index: media/filters/decoder_stream.h
diff --git a/media/filters/decoder_stream.h b/media/filters/decoder_stream.h
index b8a0d049294f8524ef7819d6c1eefbc79fdd4d59..9a6f2a0acb8dfe4bbb586e3c57c7610e77ba1263 100644
--- a/media/filters/decoder_stream.h
+++ b/media/filters/decoder_stream.h
@@ -119,6 +119,10 @@ class MEDIA_EXPORT DecoderStream {
     STATE_ERROR
   };
 
+  DemuxerStream::Type GetStreamType() {
+    return StreamType;
+  }
+
   void SelectDecoder(const SetDecryptorReadyCB& set_decryptor_ready_cb);
 
   // Called when |decoder_selector| selected the |selected_decoder|.