Oilpan: address ScrollableArea-induced build breakage from r194957.

Source/core/frame/RootFrameViewport.cpp

Index: Source/core/frame/RootFrameViewport.cpp
diff --git a/Source/core/frame/RootFrameViewport.cpp b/Source/core/frame/RootFrameViewport.cpp
index 7ae415edb2af0f3a06973bc2553f0e8acad969f6..155630db1f7c824e697c5bc41e6bbdf3ba7c1512 100644
--- a/Source/core/frame/RootFrameViewport.cpp
+++ b/Source/core/frame/RootFrameViewport.cpp
@@ -19,12 +19,6 @@ RootFrameViewport::RootFrameViewport(ScrollableArea& visualViewport, ScrollableA
 {
 }
 
-DEFINE_TRACE(RootFrameViewport)
-{
-    visitor->trace(m_visualViewport);
-    visitor->trace(m_layoutViewport);
-}
-
 void RootFrameViewport::updateScrollAnimator()
 {
     scrollAnimator()->setCurrentPosition(toFloatPoint(scrollOffsetFromScrollAnimators()));