Oilpan: add missing leftmost trace()s for GC mixins.

Source/modules/mediastream/UserMediaController.cpp

Index: Source/modules/mediastream/UserMediaController.cpp
diff --git a/Source/modules/mediastream/UserMediaController.cpp b/Source/modules/mediastream/UserMediaController.cpp
index 2a73968c182c2ec2a07fa1929fbdc9da33d5ccab..ec47bef3ef60554833e4de1dc617f4a573ba350d 100644
--- a/Source/modules/mediastream/UserMediaController.cpp
+++ b/Source/modules/mediastream/UserMediaController.cpp
@@ -39,6 +39,11 @@ UserMediaController::UserMediaController(UserMediaClient* client)
 
 DEFINE_EMPTY_DESTRUCTOR_WILL_BE_REMOVED(UserMediaController)
 
+void UserMediaController::trace(Visitor* visitor)
+{
+    WillBeHeapSupplement<LocalFrame>::trace(visitor);
+}
+
 PassOwnPtrWillBeRawPtr<UserMediaController> UserMediaController::create(UserMediaClient* client)
 {
     return adoptPtrWillBeNoop(new UserMediaController(client));