[Refactor] Introduce MediaDelegate

ash/shell/shell_delegate_impl.h

Index: ash/shell/shell_delegate_impl.h
diff --git a/ash/shell/shell_delegate_impl.h b/ash/shell/shell_delegate_impl.h
index 35f46ff1324d6b482393468be31c7b76a26c011a..efd17acd82b312d52cc1380b4260ceb302000748 100644
--- a/ash/shell/shell_delegate_impl.h
+++ b/ash/shell/shell_delegate_impl.h
@@ -46,11 +46,9 @@ class ShellDelegateImpl : public ash::ShellDelegate {
   virtual ash::SessionStateDelegate* CreateSessionStateDelegate() OVERRIDE;
   virtual ash::AccessibilityDelegate* CreateAccessibilityDelegate() OVERRIDE;
   virtual ash::NewWindowDelegate* CreateNewWindowDelegate() OVERRIDE;
+  virtual ash::MediaDelegate* CreateMediaDelegate() OVERRIDE;
   virtual aura::client::UserActionClient* CreateUserActionClient() OVERRIDE;
   virtual void RecordUserMetricsAction(UserMetricsAction action) OVERRIDE;
-  virtual void HandleMediaNextTrack() OVERRIDE;
-  virtual void HandleMediaPlayPause() OVERRIDE;
-  virtual void HandleMediaPrevTrack() OVERRIDE;
   virtual ui::MenuModel* CreateContextMenu(
       aura::Window* root_window) OVERRIDE;
   virtual RootWindowHostFactory* CreateRootWindowHostFactory() OVERRIDE;