Oilpan: reliably track listening PlatformEventControllers.

Source/modules/screen_orientation/ScreenOrientationController.cpp

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "config.h"
 #include "modules/screen_orientation/ScreenOrientationController.h"
 
 #include "core/events/Event.h"
 #include "core/frame/FrameView.h"
 #include "core/frame/LocalFrame.h"
@@ skipping 187 matching lines @@
         startUpdating();
     else
         stopUpdating();
 }
 
 void ScreenOrientationController::trace(Visitor* visitor)
 {
     visitor->trace(m_orientation);
     FrameDestructionObserver::trace(visitor);
     WillBeHeapSupplement<LocalFrame>::trace(visitor);
+    PlatformEventController::trace(visitor);
 }
 
 } // namespace blink