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 bddefd4473a70128fe5d5fad032538135422a8f5..80d0b8c6f3bab0bc655163f4a3e6b2c9a84c5387 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);
 }