Defer the real work in updateCompositingLayers until it's really needed.

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 dfa517ef274c4fc5828373c1d91ed03c8f28327c..6b9e9f4c0d6294b2158a1952c7e0bb46b2289dc9 100644
--- a/Source/core/rendering/RenderLayerScrollableArea.cpp
+++ b/Source/core/rendering/RenderLayerScrollableArea.cpp
@@ -870,6 +870,9 @@ void RenderLayerScrollableArea::positionOverflowControls(const IntSize& offsetFr
     if (m_resizer)
         m_resizer->setFrameRect(resizerCornerRect(borderBox, ResizerForPointer));
 
+    // FIXME, this should eventually be removed, once we are certain that composited
+    // controls get correctly positioned on a compositor update. For now, conservatively
+    // leaving this unchanged.
     if (m_box->compositedLayerMapping())
         m_box->compositedLayerMapping()->positionOverflowControlsLayers(offsetFromRoot);
 }