Tidy up UserMediaController references from mediastream request objects.

Source/modules/mediastream/MediaDevicesRequest.cpp

Index: Source/modules/mediastream/MediaDevicesRequest.cpp
diff --git a/Source/modules/mediastream/MediaDevicesRequest.cpp b/Source/modules/mediastream/MediaDevicesRequest.cpp
index cc436abeab88f6e2df5d999c482ef5434e839597..def9d23f3cf6170e7fdbf4c67ef42fca4c600b11 100644
--- a/Source/modules/mediastream/MediaDevicesRequest.cpp
+++ b/Source/modules/mediastream/MediaDevicesRequest.cpp
@@ -77,11 +77,12 @@ void MediaDevicesRequest::succeed(const MediaDeviceInfoVector& mediaDevices)
 void MediaDevicesRequest::stop()
 {
     m_callback.clear();
-    m_controller = 0;
+    m_controller.clear();
 }
 
 void MediaDevicesRequest::trace(Visitor* visitor)
 {
+    visitor->trace(m_controller);
     visitor->trace(m_callback);
 }