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

media/mojo/services/mojo_media_application.h

Index: media/mojo/services/mojo_media_application.h
diff --git a/media/mojo/services/mojo_media_application.h b/media/mojo/services/mojo_media_application.h
index 71b1e06527bf350fac2da53d0d0a28756a8324e7..c1e24dffa1dafc7ee41d105764dfd742304ac753 100644
--- a/media/mojo/services/mojo_media_application.h
+++ b/media/mojo/services/mojo_media_application.h
@@ -2,6 +2,7 @@
 // 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"
@@ -17,12 +18,22 @@
     : 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,
@@ -33,7 +44,11 @@
   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_;
 };