Turn audio ducking on by default on Windows again.

content/renderer/media/webrtc/peer_connection_dependency_factory.cc

 // Copyright 2014 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.
 
 #include "content/renderer/media/webrtc/peer_connection_dependency_factory.h"
 
 #include <vector>
 
 #include "base/command_line.h"
 #include "base/strings/utf_string_conversions.h"
@@ skipping 80 matching lines @@
         // set the constraint to false to later disable the software effect.
         if (is_mandatory) {
           constraints->AddMandatory(kConstraintEffectMap[i].constraint,
               webrtc::MediaConstraintsInterface::kValueFalse, true);
         } else {
           constraints->AddOptional(kConstraintEffectMap[i].constraint,
               webrtc::MediaConstraintsInterface::kValueFalse, true);
         }
         DVLOG(1) << "Disabling constraint: "
                  << kConstraintEffectMap[i].constraint;
+      } else if (kConstraintEffectMap[i].effect ==
+                 media::AudioParameters::DUCKING && value && !is_mandatory) {
+        // Special handling of the DUCKING flag that sets the optional
+        // constraint to |false| to match what the device will support.
+        constraints->AddOptional(kConstraintEffectMap[i].constraint,
+            webrtc::MediaConstraintsInterface::kValueFalse, true);
+        // No need to modify |effects| since the ducking flag is already off.
+        DCHECK((*effects & media::AudioParameters::DUCKING) == 0);
       }
     }
   }
 }
 
 class P2PPortAllocatorFactory : public webrtc::PortAllocatorFactoryInterface {
  public:
   P2PPortAllocatorFactory(
       P2PSocketDispatcher* socket_dispatcher,
       talk_base::NetworkManager* network_manager,
@@ skipping 537 matching lines @@
 }
 
 void PeerConnectionDependencyFactory::EnsureWebRtcAudioDeviceImpl() {
   if (audio_device_)
     return;
 
   audio_device_ = new WebRtcAudioDeviceImpl();
 }
 
 }  // namespace content