Oilpan: disable conservative GCs during initial GC mixin construction.

Source/core/frame/DeviceSingleWindowEventController.h

Index: Source/core/frame/DeviceSingleWindowEventController.h
diff --git a/Source/core/frame/DeviceSingleWindowEventController.h b/Source/core/frame/DeviceSingleWindowEventController.h
index 5f08fe76c443aa8f4a807406710a922c8c3e6603..1d712f0522469320390b0ed099bfb077c6f33f3a 100644
--- a/Source/core/frame/DeviceSingleWindowEventController.h
+++ b/Source/core/frame/DeviceSingleWindowEventController.h
@@ -15,7 +15,6 @@ class Document;
 class Event;
 
 class DeviceSingleWindowEventController : public NoBaseWillBeGarbageCollectedFinalized<DeviceSingleWindowEventController>, public PlatformEventController, public DOMWindowLifecycleObserver {
-    WILL_BE_USING_GARBAGE_COLLECTED_MIXIN(DeviceSingleWindowEventController);
 public:
     virtual ~DeviceSingleWindowEventController();
 
@@ -29,10 +28,10 @@ public:
     virtual void didRemoveAllEventListeners(LocalDOMWindow*) override;
 
 protected:
-    Document& document() const { return *m_document; }
-
     explicit DeviceSingleWindowEventController(Document&);
 
+    Document& document() const { return *m_document; }
+
     void dispatchDeviceEvent(const PassRefPtrWillBeRawPtr<Event>);
 
     virtual PassRefPtrWillBeRawPtr<Event> lastEvent() const = 0;