Tidy up UserMediaController references from mediastream request objects.

Source/modules/mediastream/UserMediaRequest.cpp

Index: Source/modules/mediastream/UserMediaRequest.cpp
diff --git a/Source/modules/mediastream/UserMediaRequest.cpp b/Source/modules/mediastream/UserMediaRequest.cpp
index fbdf3f7d595165899ec8a42a12b634bf682ff8c9..21a65014b50aba2534f2b9704d4b5519ce7edc55 100644
--- a/Source/modules/mediastream/UserMediaRequest.cpp
+++ b/Source/modules/mediastream/UserMediaRequest.cpp
@@ -179,7 +179,7 @@ void UserMediaRequest::contextDestroyed()
 {
     if (m_controller) {
         m_controller->cancelUserMediaRequest(this);
-        m_controller = 0;
+        m_controller = nullptr;
     }
 
     ContextLifecycleObserver::contextDestroyed();
@@ -187,6 +187,7 @@ void UserMediaRequest::contextDestroyed()
 
 void UserMediaRequest::trace(Visitor* visitor)
 {
+    visitor->trace(m_controller);
     visitor->trace(m_successCallback);
     visitor->trace(m_errorCallback);
 }