Turn audio ducking on by default on Windows again.

media/audio/win/audio_low_latency_input_win.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.
 //
 // Implementation of AudioInputStream for Windows using Windows Core Audio
 // WASAPI for low latency capturing.
 //
 // Overview of operation:
 //
 // - An object of WASAPIAudioInputStream is created by the AudioManager
@@ skipping 139 matching lines @@
   // is defined as the block of data which the user received in each
   // OnData() callback.
   size_t packet_size_frames_;
 
   // Size in bytes of each audio packet.
   size_t packet_size_bytes_;
 
   // Length of the audio endpoint buffer.
   uint32 endpoint_buffer_size_frames_;
 
-  // A copy of the supplied AudioParameter's |effects|.
-  const int effects_;
+  // A copy of the supplied AudioParameter's |effects|.  If ducking was
+  // specified (desired device=communications) but we ended up not being
+  // able to open the communications device, this flag will be cleared.
+  int effects_;
 
   // Contains the unique name of the selected endpoint device.
   // Note that AudioManagerBase::kDefaultDeviceId represents the default
   // device role and is not a valid ID as such.
   std::string device_id_;
 
   // Conversion factor used in delay-estimation calculations.
   // Converts a raw performance counter value to 100-nanosecond unit.
   double perf_count_to_100ns_units_;
 
@@ skipping 43 matching lines @@
   // Extra audio bus used for storage of deinterleaved data for the OnData
   // callback.
   scoped_ptr<media::AudioBus> audio_bus_;
 
   DISALLOW_COPY_AND_ASSIGN(WASAPIAudioInputStream);
 };
 
 }  // namespace media
 
 #endif  // MEDIA_AUDIO_WIN_AUDIO_LOW_LATENCY_INPUT_WIN_H_