Use LocalMediaStreamAudioSource for screen-casting use cases.

content/renderer/media/media_stream_audio_processor.cc

Index: content/renderer/media/media_stream_audio_processor.cc
diff --git a/content/renderer/media/media_stream_audio_processor.cc b/content/renderer/media/media_stream_audio_processor.cc
index 9b8eaf35ffc0b438b550a88999166fd3653e47ea..1815705e9208094a5c850e866e2b038f8dec6f7b 100644
--- a/content/renderer/media/media_stream_audio_processor.cc
+++ b/content/renderer/media/media_stream_audio_processor.cc
@@ -449,6 +449,42 @@ void MediaStreamAudioProcessor::OnIpcClosing() {
   aec_dump_message_filter_ = NULL;
 }
 
+// static
+bool MediaStreamAudioProcessor::ShouldRouteAudioThroughProcessor(
+    const blink::WebMediaConstraints& constraints,
+    int effects_flags) {
+  // Note: This method should by kept in-sync with any changes to the logic in
+  // MediaStreamAudioProcessor::InitializeAudioProcessingModule().

[xjz, 2016/08/13 00:43:56]

nit: The logic here is in sync with that in
MediaStreamAudioProcessor::IniitializeAudioProcessingModule() except for the
GetGoogAudioMirroring() on Line 461. Can we move this comment below that logic,
e.g.,  to Line 463, and s/This method/The following ?

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

Actually, it's perfectly in-sync. If GetGoogAudioMirroring() returns true,
MediaStreamAudioProcessor will reverse the left and right channels of the audio.
So, the method should return true because the constraints would cause the audio
signal to be modified.

+
+  const MediaAudioConstraints audio_constraints(constraints, effects_flags);
+
+  if (audio_constraints.GetGoogAudioMirroring())
+    return true;
+
+#if !defined(OS_IOS)
+  if (audio_constraints.GetEchoCancellationProperty() ||
+      audio_constraints.GetGoogAutoGainControl()) {
+    return true;
+  }
+#endif
+
+#if !defined(OS_IOS) && !defined(OS_ANDROID)
+  if (audio_constraints.GetGoogExperimentalEchoCancellation() ||
+      audio_constraints.GetGoogTypingNoiseDetection()) {
+    return true;
+  }
+#endif
+
+  if (audio_constraints.GetGoogNoiseSuppression() ||
+      audio_constraints.GetGoogExperimentalNoiseSuppression() ||
+      audio_constraints.GetGoogBeamforming() ||
+      audio_constraints.GetGoogHighpassFilter()) {
+    return true;
+  }
+
+  return false;
+}
+
 void MediaStreamAudioProcessor::OnPlayoutData(media::AudioBus* audio_bus,
                                               int sample_rate,
                                               int audio_delay_milliseconds) {
@@ -538,8 +574,16 @@ void MediaStreamAudioProcessor::InitializeAudioProcessingModule(
   if (!echo_cancellation && !goog_experimental_aec && !goog_ns &&
       !goog_high_pass_filter && !goog_typing_detection &&
       !goog_agc && !goog_experimental_ns && !goog_beamforming) {
+    // Ensure ShouldRouteAudioThroughProcessor() is kept in-sync with all of the
+    // above logic.
+    DCHECK_EQ(audio_mirroring_, ShouldRouteAudioThroughProcessor(

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

See the comment above: it's confusing when ShouldRouteAudioThroughProcessor() is
true, but AUDIO_PROCESSING_DISABLED is set. What is the actual meaning of this
"Should..." method then?

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

I made some clarifications on these comments too. Also, I've renamed the
ShouldRouteAudioThroughProcessor() function to WouldModifyAudio() and updated
the header file comments.

+                                    constraints, input_params.effects));
     RecordProcessingState(AUDIO_PROCESSING_DISABLED);
     return;
+  } else {
+    // Ensure ShouldRouteAudioThroughProcessor() is kept in-sync with all of the
+    // above logic.
+    DCHECK(ShouldRouteAudioThroughProcessor(constraints, input_params.effects));
   }
 
   // Experimental options provided at creation.