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

media/audio/win/audio_manager_win.h

Index: media/audio/win/audio_manager_win.h
diff --git a/media/audio/win/audio_manager_win.h b/media/audio/win/audio_manager_win.h
index c2ac8d7a1c8bbebfab45a864f885c899a0bc1893..1bbfc23fddbd4f731405e0577558b489101a4050 100644
--- a/media/audio/win/audio_manager_win.h
+++ b/media/audio/win/audio_manager_win.h
@@ -56,7 +56,6 @@ class MEDIA_EXPORT AudioManagerWin : public AudioManagerBase {
 
  private:
   enum EnumerationType {
-    kUninitializedEnumeration = 0,
     kMMDeviceEnumeration,
     kWaveEnumeration,
   };
@@ -70,6 +69,10 @@ class MEDIA_EXPORT AudioManagerWin : public AudioManagerBase {
     enumeration_type_ = type;
   }
 
+  inline bool core_audio_supported() const {
+    return enumeration_type_ == kMMDeviceEnumeration;
+  }
+
   // Returns a PCMWaveInAudioInputStream instance or NULL on failure.
   // This method converts MMDevice-style device ID to WaveIn-style device ID if
   // necessary.