Turn audio ducking on by default on Windows again.

content/renderer/media/webrtc/peer_connection_dependency_factory.cc

Index: content/renderer/media/webrtc/peer_connection_dependency_factory.cc
diff --git a/content/renderer/media/webrtc/peer_connection_dependency_factory.cc b/content/renderer/media/webrtc/peer_connection_dependency_factory.cc
index f2f7f1d109b24b975286dfeeea9a6e48fa2de036..92ea66bde071e455d0af997e4f958a7df71f3425 100644
--- a/content/renderer/media/webrtc/peer_connection_dependency_factory.cc
+++ b/content/renderer/media/webrtc/peer_connection_dependency_factory.cc
@@ -98,6 +98,14 @@ void HarmonizeConstraintsAndEffects(RTCMediaConstraints* constraints,
         }
         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,

[no longer working on chromium, 2014/07/08 10:42:35]

FYI, this HarmonizeConstraintsAndEffects() method is used to modify
|constraints|, which will be passed to libjingle. Since ducking is handled by
chrome, there is no use to modify the value here.
But Setting the value here does not hurt anything. Feel free to keep what it is.

[tommi (sloooow) - chröme, 2014/07/08 12:30:56]

This actually is also for Chrome via CreateAudioCapturer (see below).

+            webrtc::MediaConstraintsInterface::kValueFalse, true);
+        // No need to modify |effects| since the ducking flag is already off.
+        DCHECK((*effects & media::AudioParameters::DUCKING) == 0);
       }
     }
   }