Add audio_device_id in mojo media ServiceFactory CreateRenderer api.

media/mojo/interfaces/service_factory.mojom

 // Copyright 2015 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.
 
 module media.mojom;
 
 import "media/mojo/interfaces/audio_decoder.mojom";
 import "media/mojo/interfaces/content_decryption_module.mojom";
 import "media/mojo/interfaces/renderer.mojom";
 import "media/mojo/interfaces/video_decoder.mojom";
 
 // A factory interface to create media services. Renderers can only access
 // ContentDecryptionModules created in the same factory.
 interface ServiceFactory {
   // Creates an AudioDecoder service.
   CreateAudioDecoder(AudioDecoder& audio_decoder);
 
   // Creates a VideoDecoder service.
   CreateVideoDecoder(VideoDecoder& video_decoder);
 
   // Creates a Renderer service.
-  CreateRenderer(Renderer& renderer);
+  // Audio stream will be played on device_id, which is defined in
+  // media/audio/audio_device_description.h. If audio_device_id
+  // is null, kDefaultDeviceId will be used. 

[xhwang, 2016/06/02 06:09:29]

I wasn't aware that in C++ code when the device ID is empty then we'll choose
the default device. In this case, maybe we should do the same on the mojo
interface. Basically drop the "?" part to make this parameter mandatory, and
update this comment s/null/empty. 

Then we can also get rid of the use of mojo::String(nullptr) which seems a bit
odd.

WDYT?

[tianyuwang1, 2016/06/02 21:50:53]

Done.

+  CreateRenderer(Renderer& renderer,
+                 string? audio_device_id);

[xhwang, 2016/06/02 06:09:29]

+alop: Just FYI, this will make it harder for us to get rid of this mojom
interface now. One solution is to add a 

Create(audio_device_id) -> (bool success) 

on the Renderer interface, so that the caller will do:

ConnectToInterface(GetProxy(&renderer));
renderer->Create(create_cb);

We have similar issues on the VideoDecoder mojo interface:
https://code.google.com/p/chromium/codesearch#chromium/src/media/mojo/interfa...

[alokp, 2016/06/02 10:54:15]

Hmm. It would be weird to call Create on an already created Interface . It would
be nice if ConnectToInterface allowed a struct to be passed to the Interface
implementation. At the same time I also do not like an Initialize method on an
abstract interface.

I do not have a better suggestion at this time. I will keep thinking about this.

 
   // Creates a ContentDecryptionModule service.
   CreateCdm(ContentDecryptionModule& cdm);
 };