Make MediaSession per frame as an attribute of Navigator

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

+/*
+ * mediasessionservice-mock contains a mock implementation of MediaSessionService.
+ */
+
+"use strict";
+
+let mediaSessionServiceMock = loadMojoModules(
+    'mediaSessionServiceMock',
+    ['third_party/WebKit/public/platform/modules/mediasession/media_session.mojom',
+     'mojo/public/js/router',
+    ]).then(mojo => {
+      let [mediaSessionService, router] = mojo.modules;
+
+      class MediaSessionServiceMock {
+        constructor(interfaceProvider) {
+          interfaceProvider.addInterfaceOverrideForTesting(
+              mediaSessionService.MediaSessionService.name,
+              handle => this.connectMediaSessionService_(handle));
+          this.interfaceProvider_ = interfaceProvider;
+          this.pendingResponse_ = null;
+        }
+
+        connectMediaSessionService_(handle) {
+          this.mediaSessionServiceStub_ = new mediaSessionService.MediaSessionService.stubClass(this);
+          this.mediaSessionServiceRouter_ = new router.Router(handle);
+          this.mediaSessionServiceRouter_.setIncomingReceiver(this.mediaSessionServiceStub_);
+        }
+
+        setMetadata(metadata) {
+          if (!!this.setMetadataStub) {

[mlamouri (slow - plz ping), 2016/10/10 20:30:34]

That's odd. Did you copy this behaviour from another mojo service mock? It
sounds that having real methods in the mock would be better like
`setMetadataCallback`.

[Zhiqiang Zhang (Slow), 2016/10/11 10:56:04]

Done.

+            this.setMetadataStub(metadata);
+          }
+        }
+      }
+
+      return new MediaSessionServiceMock(mojo.frameInterfaces);
+    });