Clean up code to use VideoFrameMetadata::MergeMetadataFrom() when appropriate.

content/renderer/media/webmediaplayer_ms_compositor.cc

Index: content/renderer/media/webmediaplayer_ms_compositor.cc
diff --git a/content/renderer/media/webmediaplayer_ms_compositor.cc b/content/renderer/media/webmediaplayer_ms_compositor.cc
index 784f0cfda2b12bab081559c5ea6f09ca6eb86b01..86e1a896411438311abd59784c47512d62d121c0 100644
--- a/content/renderer/media/webmediaplayer_ms_compositor.cc
+++ b/content/renderer/media/webmediaplayer_ms_compositor.cc
@@ -117,9 +117,7 @@ scoped_refptr<media::VideoFrame> CopyFrame(
   }
 
   // Transfer metadata keys.
-  base::DictionaryValue original_metadata;
-  frame->metadata()->MergeInternalValuesInto(&original_metadata);
-  new_frame->metadata()->MergeInternalValuesFrom(original_metadata);
+  new_frame->metadata()->MergeMetadataFrom(frame->metadata());
   return new_frame;
 }