Use LocalMediaStreamAudioSource for screen-casting use cases.

content/renderer/media/media_stream_audio_processor.h

 // Copyright 2013 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_MEDIA_STREAM_AUDIO_PROCESSOR_H_
 #define CONTENT_RENDERER_MEDIA_MEDIA_STREAM_AUDIO_PROCESSOR_H_
 
 #include "base/atomicops.h"
 #include "base/files/file.h"
 #include "base/gtest_prod_util.h"
@@ skipping 92 matching lines @@
 
   // Accessor to check if the audio processing is enabled or not.
   bool has_audio_processing() const { return audio_processing_ != NULL; }
 
   // AecDumpMessageFilter::AecDumpDelegate implementation.
   // Called on the main render thread.
   void OnAecDumpFile(const IPC::PlatformFileForTransit& file_handle) override;
   void OnDisableAecDump() override;
   void OnIpcClosing() override;
 
+  // Returns true if MediaStreamAudioProcessor would modify the audio signal,
+  // based on the |constraints| and |effects_flags| parsed from a user media
+  // request. If the audio signal would not be modified, there is no need to
+  // instantiate a MediaStreamAudioProcessor and feed audio through it. Doing so
+  // would waste a non-trivial amount of memory and CPU resources.
+  //
+  // See media::AudioParameters::PlatformEffectsMask for interpretation of
+  // |effects_flags|.
+  static bool WouldModifyAudio(const blink::WebMediaConstraints& constraints,
+                               int effects_flags);
+
  protected:
   ~MediaStreamAudioProcessor() override;
 
  private:
   friend class MediaStreamAudioProcessorTest;
 
   FRIEND_TEST_ALL_PREFIXES(MediaStreamAudioProcessorTest,
                            GetAecDumpMessageFilter);
 
   // WebRtcPlayoutDataSource::Sink implementation.
@@ skipping 92 matching lines @@
   // Object for logging UMA stats for echo information when the AEC is enabled.
   // Accessed on the main render thread.
   std::unique_ptr<EchoInformation> echo_information_;
 
   DISALLOW_COPY_AND_ASSIGN(MediaStreamAudioProcessor);
 };
 
 }  // namespace content
 
 #endif  // CONTENT_RENDERER_MEDIA_MEDIA_STREAM_AUDIO_PROCESSOR_H_