Reland of land "media: Refactor MojoMediaClient" with fix.

media/mojo/services/mojo_media_application.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+#include "base/callback.h"
 #include "base/memory/ref_counted.h"
 #include "media/mojo/interfaces/service_factory.mojom.h"
 #include "mojo/shell/public/cpp/interface_factory_impl.h"
 #include "mojo/shell/public/cpp/shell_client.h"
 #include "url/gurl.h"
 
 namespace media {
 
 class MediaLog;
 class MojoMediaClient;
 
 class MojoMediaApplication
     : public mojo::ShellClient,
       public mojo::InterfaceFactory<interfaces::ServiceFactory> {
  public:
+  // Callback to create a MojoMediaClient.
+  using CreateMojoMediaClientCB = base::Callback<scoped_ptr<MojoMediaClient>()>;
+
+  // Creates MojoMediaApplication using the default MojoMediaClient.
   static scoped_ptr<mojo::ShellClient> CreateApp();
 
-  explicit MojoMediaApplication(scoped_ptr<MojoMediaClient> mojo_media_client);
+  // Creates MojoMediaApplication using the MojoMediaClient provided by
+  // |create_mojo_media_client_cb|.
+  static scoped_ptr<mojo::ShellClient> CreateAppWithClient(
+      const CreateMojoMediaClientCB& create_mojo_media_client_cb);
+
   ~MojoMediaApplication() final;
 
  private:
+  explicit MojoMediaApplication(scoped_ptr<MojoMediaClient> mojo_media_client);
+
   // mojo::ShellClient implementation.
   void Initialize(mojo::Shell* shell,
                   const std::string& url,
                   uint32_t id) final;
   bool AcceptConnection(mojo::Connection* connection) final;
 
   // mojo::InterfaceFactory<interfaces::ServiceFactory> implementation.
   void Create(mojo::Connection* connection,
               mojo::InterfaceRequest<interfaces::ServiceFactory> request) final;
 
+  // Note: Since each instance runs on a different thread, do not share a common
+  // MojoMediaClient with other instances to avoid threading issues. Hence using
+  // a scoped_ptr here.
   scoped_ptr<MojoMediaClient> mojo_media_client_;
+
   mojo::Shell* shell_;
   scoped_refptr<MediaLog> media_log_;
 };
 
 }  // namespace media