Media Session API: make MediaMetadata mutable.

third_party/WebKit/LayoutTests/media/mediasession/mojo/resources/utils.js

 function assert_image_equals(expected, observed) {
   assert_equals(expected.src, observed.src);
   assert_equals(expected.type, observed.type);
   assert_equals(expected.sizes, observed.sizes);
 }
 
 function assert_metadata_equals(expected, observed) {
-  assert_equals(expected.title, observed.title);
-  assert_equals(expected.artist, observed.artist);
-  assert_equals(expected.album, observed.album);
-  assert_equals(expected.artwork.length, observed.artwork.length);
-  for (var i = 0; i < expected.artwork.length; i++) {
+  assert_equals(expected.title, observed.title, 'metadata.title');
+  assert_equals(expected.artist, observed.artist, 'metadata.artist');
+  assert_equals(expected.album, observed.album, 'metadata.album');
+  assert_equals(expected.artwork.length, observed.artwork.length,
+                'metadata.artwork.length');
+  for (var i = 0; i < expected.artwork.length; i++)
     assert_image_equals(expected.artwork[i], observed.artwork[i]);
-  }
 }