Adding mojo MediaSessionClient to support media controls

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

Index: third_party/WebKit/LayoutTests/media/mediasession/mojo/resources/mediasessionservice-mock.js
diff --git a/third_party/WebKit/LayoutTests/media/mediasession/mojo/resources/mediasessionservice-mock.js b/third_party/WebKit/LayoutTests/media/mediasession/mojo/resources/mediasessionservice-mock.js
index 833db371d64361a2c46db7508c08f616902aad6b..f02422a5368d9b68f1477a49be1d57b9235f2e41 100644
--- a/third_party/WebKit/LayoutTests/media/mediasession/mojo/resources/mediasessionservice-mock.js
+++ b/third_party/WebKit/LayoutTests/media/mediasession/mojo/resources/mediasessionservice-mock.js
@@ -66,6 +66,29 @@ let mediaSessionServiceMock = loadMojoModules(
         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);