Encrypted Media: Add TRACE_EVENT to trace eme implementation performance.

media/filters/decrypting_video_decoder.h

Index: media/filters/decrypting_video_decoder.h
diff --git a/media/filters/decrypting_video_decoder.h b/media/filters/decrypting_video_decoder.h
index 900b0bb3db9e8a1c74b620bea286329df2d5fc85..d2fb04ba3ab8ca6f6273d2effe58ab14b4b9e42d 100644
--- a/media/filters/decrypting_video_decoder.h
+++ b/media/filters/decrypting_video_decoder.h
@@ -155,6 +155,10 @@ class MEDIA_EXPORT DecryptingVideoDecoder : public VideoDecoder {
   // decryption key.
   bool key_added_while_pending_decode_;
 
+  // A unique ID to trace Decryptor::DecryptAndDecodeVideo() call and the
+  // matching DecryptCB call (in DoDeliverFrame()).
+  uint32 trace_id_;
+
   DISALLOW_COPY_AND_ASSIGN(DecryptingVideoDecoder);
 };