Adding mojo MediaSessionClient to support media controls

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

+var MOJOM_ACTION_PLAY = 0;
+var MOJOM_ACTION_PAUSE = 1;
+var MOJOM_ACTION_PLAY_PAUSE = 2;
+var MOJOM_ACTION_PREVIOUS_TRACK = 3;
+var MOJOM_ACTION_NEXT_TRACK = 4;
+var MOJOM_ACTION_SEEK_FORWARD = 5;
+var MOJOM_ACTION_SEEK_BACKWARD = 6;
+
 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_image_equals(expected.artwork[i], observed.artwork[i]);
   }
 }