Use LocalMediaStreamAudioSource for screen-casting use cases.

content/renderer/media/local_media_stream_audio_source.h

Index: content/renderer/media/local_media_stream_audio_source.h
diff --git a/content/renderer/media/local_media_stream_audio_source.h b/content/renderer/media/local_media_stream_audio_source.h
new file mode 100644
index 0000000000000000000000000000000000000000..1d9d63a0e8d2043c38333e6e5d5a6a077f634cfe
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+++ b/content/renderer/media/local_media_stream_audio_source.h
@@ -0,0 +1,58 @@
+// Copyright 2016 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.
+
+#ifndef CONTENT_RENDERER_MEDIA_LOCAL_MEDIA_STREAM_AUDIO_SOURCE_H_
+#define CONTENT_RENDERER_MEDIA_LOCAL_MEDIA_STREAM_AUDIO_SOURCE_H_
+
+#include "content/renderer/media/media_stream_audio_source.h"
+#include "media/base/audio_capturer_source.h"
+
+namespace content {
+
+// Represents a local source of audio data generated by an AudioInputDevice.
+// Uses content::AudioDeviceFactory to auto-create the AudioInputDevice, using
+// the parameters and session ID found in StreamDeviceInfo, just before the
+// first track is connected. Audio data is transported directly to the tracks
+// (i.e., there is no audio processing).
+class CONTENT_EXPORT LocalMediaStreamAudioSource

[o1ka, 2016/08/15 15:15:00]

Should probably the naming match ProcessedLocalAudioSource? Like,
"UnprocessedLocalAudioSource"?

[miu, 2016/08/16 04:30:02]

I don't know if it's necessary to call-out in its name the fact that this
implementation doesn't process the audio. Unless you feel super-strongly about
it, IMHO, the current naming is accurate (because this is a
MediaStreamAudioSource of locally-generated audio).

[o1ka, 2016/08/16 14:18:11]

I just find it easier to read the code when naming reflects inheritance
hierarchy. (ProcessedLocalMediaStreamAudioSource is unfortunately too long :) )
No, I don't feel super-strongly, just a little annoyed that I have to embrace
and memorize that those classes are siblings :)

[miu, 2016/08/16 20:13:39]

I can understand this concern. FWIW, the other two implementations of
MediaStreamAudioSource, ExternalMediaStreamAudioSource and
PeerConnectionRemoteAudioSource, don't do processing either. So, if anything,
the ProcessedLocalAudioSource is the only "special" one to remember. ;-)

+    : NON_EXPORTED_BASE(public MediaStreamAudioSource),
+      NON_EXPORTED_BASE(public media::AudioCapturerSource::CaptureCallback) {
+ public:
+  // |consumer_render_frame_id| references the RenderFrame that will consume the
+  // audio data. Audio parameters and (optionally) a pre-existing audio session
+  // ID are read from |device_info|.
+  LocalMediaStreamAudioSource(int consumer_render_frame_id,
+                              const StreamDeviceInfo& device_info);
+
+  ~LocalMediaStreamAudioSource() final;
+
+ private:
+  // MediaStreamAudioSource implementation.
+  bool EnsureSourceIsStarted() final;
+  void EnsureSourceIsStopped() final;
+
+  // media::AudioCapturerSource::CaptureCallback implementation.
+  void Capture(const media::AudioBus* audio_bus,
+               int audio_delay_milliseconds,
+               double volume,
+               bool key_pressed) final;
+  void OnCaptureError(const std::string& message) final;
+
+  // The RenderFrame that will consume the audio data. Used when creating
+  // AudioInputDevices via the AudioDeviceFactory.
+  const int consumer_render_frame_id_;
+
+  // The device created by the AudioDeviceFactory in EnsureSourceIsStarted().
+  scoped_refptr<media::AudioCapturerSource> source_;
+
+  // In debug builds, check that all methods that could cause object graph
+  // or data flow changes are being called on the main thread.
+  base::ThreadChecker thread_checker_;
+
+  DISALLOW_COPY_AND_ASSIGN(LocalMediaStreamAudioSource);
+};
+
+}  // namespace content
+
+#endif  // CONTENT_RENDERER_MEDIA_LOCAL_MEDIA_STREAM_AUDIO_SOURCE_H_