Move the Widget hierarchy to the Oilpan heap.

Source/core/rendering/RenderLayerScrollableArea.cpp

Index: Source/core/rendering/RenderLayerScrollableArea.cpp
diff --git a/Source/core/rendering/RenderLayerScrollableArea.cpp b/Source/core/rendering/RenderLayerScrollableArea.cpp
index 308e7016e74a1e8f5cd0b1f9acb5855104f009e1..07d732626f6e91b819074c3a11cae6bbcecb1ce2 100644
--- a/Source/core/rendering/RenderLayerScrollableArea.cpp
+++ b/Source/core/rendering/RenderLayerScrollableArea.cpp
@@ -357,7 +357,7 @@ void RenderLayerScrollableArea::setScrollOffset(const IntPoint& newScrollOffset)
     LocalFrame* frame = box().frame();
     ASSERT(frame);
 
-    RefPtr<FrameView> frameView = box().frameView();
+    RefPtrWillBeRawPtr<FrameView> frameView = box().frameView();
 
     TRACE_EVENT1(TRACE_DISABLED_BY_DEFAULT("devtools.timeline"), "ScrollLayer", "data", InspectorScrollLayerEvent::data(&box()));
     // FIXME(361045): remove InspectorInstrumentation calls once DevTools Timeline migrates to tracing.
@@ -849,9 +849,9 @@ static inline RenderObject* rendererForScrollbar(RenderObject& renderer)
     return &renderer;
 }
 
-PassRefPtr<Scrollbar> RenderLayerScrollableArea::createScrollbar(ScrollbarOrientation orientation)
+PassRefPtrWillBeRawPtr<Scrollbar> RenderLayerScrollableArea::createScrollbar(ScrollbarOrientation orientation)
 {
-    RefPtr<Scrollbar> widget;
+    RefPtrWillBeRawPtr<Scrollbar> widget = nullptr;
     RenderObject* actualRenderer = rendererForScrollbar(box());
     bool hasCustomScrollbarStyle = actualRenderer->isBox() && actualRenderer->style()->hasPseudoStyle(SCROLLBAR);
     if (hasCustomScrollbarStyle) {
@@ -872,7 +872,7 @@ PassRefPtr<Scrollbar> RenderLayerScrollableArea::createScrollbar(ScrollbarOrient
 
 void RenderLayerScrollableArea::destroyScrollbar(ScrollbarOrientation orientation)
 {
-    RefPtr<Scrollbar>& scrollbar = orientation == HorizontalScrollbar ? m_hBar : m_vBar;
+    RefPtrWillBePersistent<Scrollbar>& scrollbar = orientation == HorizontalScrollbar ? m_hBar : m_vBar;
     if (!scrollbar)
         return;