Add basic support for "googDucking" to getUserMedia on Windows.

media/audio/audio_parameters.h

Index: media/audio/audio_parameters.h
diff --git a/media/audio/audio_parameters.h b/media/audio/audio_parameters.h
index 62ff4fd48f1eb1a3098b46491e254af67c0dbf89..b65f64fc678efefabc7e4c8f77da79fa96422d05 100644
--- a/media/audio/audio_parameters.h
+++ b/media/audio/audio_parameters.h
@@ -48,7 +48,8 @@ class MEDIA_EXPORT AudioParameters {
   // effects should be enabled.
   enum PlatformEffectsMask {
     NO_EFFECTS = 0x0,
-    ECHO_CANCELLER = 0x1
+    ECHO_CANCELLER = 0x1,
+    DUCKING = 0x2,  // Enables ducking if the OS supports it.
   };
 
   AudioParameters();