NOT FOR LANDING Introduce and use WebMediaSession

Source/core/loader/EmptyClients.h

Index: Source/core/loader/EmptyClients.h
diff --git a/Source/core/loader/EmptyClients.h b/Source/core/loader/EmptyClients.h
index c3afb1636c4304469ae6d8ab378365e33a2ec341..7dfaf551a758e9322e8443020d9d429d20962cd1 100644
--- a/Source/core/loader/EmptyClients.h
+++ b/Source/core/loader/EmptyClients.h
@@ -233,6 +233,7 @@ public:
     PassRefPtrWillBeRawPtr<Widget> createPlugin(HTMLPlugInElement*, const KURL&, const Vector<String>&, const Vector<String>&, const String&, bool, DetachedPluginPolicy) override;
     bool canCreatePluginWithoutRenderer(const String& mimeType) const override { return false; }
     PassOwnPtr<WebMediaPlayer> createWebMediaPlayer(HTMLMediaElement&, const WebURL&, WebMediaPlayerClient*) override;
+    PassOwnPtr<WebMediaSession> createWebMediaSession() override;
 
     ObjectContentType objectContentType(const KURL&, const String&, bool) override { return ObjectContentType(); }