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

content/renderer/media/media_stream_audio_processor_options.cc

Index: content/renderer/media/media_stream_audio_processor_options.cc
diff --git a/content/renderer/media/media_stream_audio_processor_options.cc b/content/renderer/media/media_stream_audio_processor_options.cc
index 882d57c8c7bb62691615b2d0327faebeded69883..f70c0900e2532079568527bda19803b81ecbec1c 100644
--- a/content/renderer/media/media_stream_audio_processor_options.cc
+++ b/content/renderer/media/media_stream_audio_processor_options.cc
@@ -8,6 +8,7 @@
 #include "base/logging.h"
 #include "base/path_service.h"
 #include "base/strings/utf_string_conversions.h"
+#include "content/common/media/media_stream_options.h"
 #include "content/renderer/media/rtc_media_constraints.h"
 #include "media/audio/audio_parameters.h"
 #include "third_party/WebKit/public/platform/WebMediaConstraints.h"
@@ -41,6 +42,10 @@ struct {
     webrtc::MediaConstraintsInterface::kValueTrue },
   { webrtc::MediaConstraintsInterface::kTypingNoiseDetection,
     webrtc::MediaConstraintsInterface::kValueTrue },
+#if defined(OS_WIN)
+  { content::kMediaStreamAudioDucking,
+    webrtc::MediaConstraintsInterface::kValueTrue },
+#endif
 };
 
 } // namespace
@@ -50,6 +55,12 @@ void ApplyFixedAudioConstraints(RTCMediaConstraints* constraints) {
     bool already_set_value;
     if (!webrtc::FindConstraint(constraints, kDefaultAudioConstraints[i].key,
                                 &already_set_value, NULL)) {
+      // TODO(tommi): This adds the default constraints to the 'mandatory'
+      // section but it might make more sense to add these to the optional
+      // section since if for whatever reason the default implied constraints
+      // aren't supported on some platform then we should not fail the
+      // getUserMedia call.  'kMediaStreamAudioDucking' is an example where
+      // it is not supported on XP but is supported on newer versions.
       constraints->AddMandatory(kDefaultAudioConstraints[i].key,
           kDefaultAudioConstraints[i].value, false);
     } else {