Gracefully support Navigator Gamepad methods in a detached state.

Source/modules/device_orientation/DeviceMotionController.h

Index: Source/modules/device_orientation/DeviceMotionController.h
diff --git a/Source/modules/device_orientation/DeviceMotionController.h b/Source/modules/device_orientation/DeviceMotionController.h
index 266ff0ffb04c82af03e262b047325f8be2d420aa..2c43db6c3aa460545deadc051af7ea6279cf4982 100644
--- a/Source/modules/device_orientation/DeviceMotionController.h
+++ b/Source/modules/device_orientation/DeviceMotionController.h
@@ -62,6 +62,9 @@ private:
     virtual bool hasLastData() OVERRIDE;
     virtual PassRefPtrWillBeRawPtr<Event> getLastEvent() OVERRIDE;
     virtual bool isNullEvent(Event*) OVERRIDE;
+    virtual Document* document() OVERRIDE;
+
+    Document& m_document;
 };
 
 } // namespace WebCore