Adding mojo MediaSessionClient to support media controls

third_party/WebKit/LayoutTests/media/mediasession/mojo/resources/mediasessionservice-mock.js

 /*
  * mediasessionservice-mock contains a mock implementation of MediaSessionService.
  */
 
 "use strict";
 
 function mojoString16ToJS(mojoString16) {
   return String.fromCharCode.apply(null, mojoString16.data);
 }
 
@@ skipping 48 matching lines @@
         }
 
         setMetadata(metadata) {
           if (!!this.metadataCallback_)
             this.metadataCallback_(mojoMetadataToJS(metadata));
         }
 
         setMetadataCallback(callback) {
           this.metadataCallback_ = callback;
         }
+
+        didSetEventHandlerForAction(action, is_set) {
+          if (!!this.eventHandlerCallback_)
+            this.eventHandlerCallback_(action, is_set);
+        }
+
+        setEventHandlerCallback(callback) {
+          this.eventHandlerCallback_ = callback;
+        }
+
+        setClient(client) {
+          this.client_ = client;
+          if (!!this.clientCallback_)
+            this.clientCallback_();
+        }
+
+        setClientCallback(callback) {
+          this.clientCallback_ = callback;
+        }
+
+        getClient() {
+          return this.client_;
+        }
       }
 
       return new MediaSessionServiceMock(mojo.frameInterfaces);
     });