Don't run WSOLA over always muted channels.

media/renderers/audio_renderer_impl.h

 // Copyright (c) 2012 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.
 
 // Audio rendering unit utilizing an AudioRendererSink to output data.
 //
 // This class lives inside three threads during it's lifetime, namely:
 // 1. Render thread
 //    Where the object is created.
 // 2. Media thread (provided via constructor)
@@ skipping 216 matching lines @@
   std::unique_ptr<base::TickClock> tick_clock_;
 
   // Memory usage of |algorithm_| recorded during the last
   // HandleDecodedBuffer_Locked() call.
   int64_t last_audio_memory_usage_;
 
   // Sample rate of the last decoded audio buffer. Allows for detection of
   // sample rate changes due to implicit AAC configuration change.
   int last_decoded_sample_rate_;
 
+  // Indicates which channels are muted and can be ignored by the algorithm.
+  std::vector<bool> channel_mask_;
+
   // After Initialize() has completed, all variables below must be accessed
   // under |lock_|. ------------------------------------------------------------
   base::Lock lock_;
 
   // Algorithm for scaling audio.
   double playback_rate_;
   std::unique_ptr<AudioRendererAlgorithm> algorithm_;
 
   // Simple state tracking variable.
   State state_;
@@ skipping 43 matching lines @@
 
   // NOTE: Weak pointers must be invalidated before all other member variables.
   base::WeakPtrFactory<AudioRendererImpl> weak_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(AudioRendererImpl);
 };
 
 }  // namespace media
 
 #endif  // MEDIA_RENDERERS_AUDIO_RENDERER_IMPL_H_