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

Source/core/rendering/RenderLayer.cpp

Index: Source/core/rendering/RenderLayer.cpp
diff --git a/Source/core/rendering/RenderLayer.cpp b/Source/core/rendering/RenderLayer.cpp
index 2dcfcb143440260a5a7b2eb8ab6423ffb46c79f1..1d75a7fe37fbbde2c761f0f62166ff0d2f335d1f 100644
--- a/Source/core/rendering/RenderLayer.cpp
+++ b/Source/core/rendering/RenderLayer.cpp
@@ -701,20 +701,6 @@ void RenderLayer::updateLayerPositionsAfterScroll(RenderGeometryMap* geometryMap
         geometryMap->popMappingsToAncestor(parent());
 }
 
-void RenderLayer::positionNewlyCreatedOverflowControls()
-{
-    if (!compositedLayerMapping()->hasUnpositionedOverflowControlsLayers())
-        return;
-
-    RenderGeometryMap geometryMap(UseTransforms);
-    RenderView* view = renderer()->view();
-    if (this != view->layer() && parent())
-        geometryMap.pushMappingsToAncestor(parent(), 0);
-
-    LayoutPoint offsetFromRoot = LayoutPoint(geometryMap.absolutePoint(FloatPoint()));
-    positionOverflowControls(toIntSize(roundedIntPoint(offsetFromRoot)));
-}
-
 bool RenderLayer::hasBlendMode() const
 {
     return RuntimeEnabledFeatures::cssCompositingEnabled() && renderer()->hasBlendMode();
@@ -2041,7 +2027,7 @@ void RenderLayer::didUpdateNeedsCompositedScrolling()
 
     dirtyStackingContainerZOrderLists();
 
-    compositor()->setShouldReevaluateCompositingAfterLayout();
+    compositor()->setNeedsToRecomputeCompositingRequirements();
     compositor()->setCompositingLayersNeedRebuild();
 }
 
@@ -4566,7 +4552,7 @@ void RenderLayer::dirtyZOrderLists()
         compositor()->setNeedsUpdateCompositingRequirementsState();
         compositor()->setCompositingLayersNeedRebuild();
         if (acceleratedCompositingForOverflowScrollEnabled())
-            compositor()->setShouldReevaluateCompositingAfterLayout();
+            compositor()->setNeedsToRecomputeCompositingRequirements();
     }
 }
 
@@ -4603,7 +4589,7 @@ void RenderLayer::dirtyNormalFlowList()
     if (!renderer()->documentBeingDestroyed()) {
         compositor()->setCompositingLayersNeedRebuild();
         if (acceleratedCompositingForOverflowScrollEnabled())
-            compositor()->setShouldReevaluateCompositingAfterLayout();
+            compositor()->setNeedsToRecomputeCompositingRequirements();
     }
 }
 
@@ -5355,6 +5341,17 @@ IntRect RenderLayer::scrollCornerAndResizerRect() const
     return m_scrollableArea->scrollCornerAndResizerRect();
 }
 
+void RenderLayer::positionOverflowControls()
+{
+    RenderGeometryMap geometryMap(UseTransforms);
+    RenderView* view = renderer()->view();
+    if (this != view->layer() && parent())
+        geometryMap.pushMappingsToAncestor(parent(), 0);
+
+    LayoutPoint offsetFromRoot = LayoutPoint(geometryMap.absolutePoint(FloatPoint()));
+    positionOverflowControls(toIntSize(roundedIntPoint(offsetFromRoot)));
+}
+
 void RenderLayer::positionOverflowControls(const IntSize& offsetFromRoot)
 {
     if (m_scrollableArea)