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

Source/core/rendering/RenderLayerCompositor.h

Index: Source/core/rendering/RenderLayerCompositor.h
diff --git a/Source/core/rendering/RenderLayerCompositor.h b/Source/core/rendering/RenderLayerCompositor.h
index 8dc2b8c04b5d1cd9ec2dc8c0ecdcd0081385296d..d70d024807d9bfb7d473bc3955daec9d487015dc 100644
--- a/Source/core/rendering/RenderLayerCompositor.h
+++ b/Source/core/rendering/RenderLayerCompositor.h
@@ -46,7 +46,8 @@ enum CompositingUpdateType {
     CompositingUpdateAfterStyleChange,
     CompositingUpdateAfterLayout,
     CompositingUpdateOnScroll,
-    CompositingUpdateOnCompositedScroll
+    CompositingUpdateOnCompositedScroll,
+    CompositingUpdateOnBeginDrawingFrame
 };
 
 // RenderLayerCompositor manages the hierarchy of
@@ -199,7 +200,7 @@ public:
     void resetTrackedRepaintRects();
     void setTracksRepaints(bool);
 
-    void setShouldReevaluateCompositingAfterLayout() { m_reevaluateCompositingAfterLayout = true; }
+    void setShouldReevaluateCompositingAfterLayout() { m_needsToRecomputeCompositingRequirements = true; }

[Ian Vollick, 2013/10/08 18:32:21]

nit: should this function be renamed as well, then?

 
     // Returns all reasons (direct, indirectly due to subtree, and indirectly due to overlap) that a layer should be composited.
     CompositingReasons reasonsForCompositing(const RenderLayer*) const;
@@ -316,9 +317,11 @@ private:
     int m_compositedLayerCount;
     bool m_showRepaintCounter;
 
+    // FIXME update this comment before landing the patch
     // When true, we have to wait until layout has happened before we can decide whether to enter compositing mode,
     // because only then do we know the final size of plugins and iframes.
-    mutable bool m_reevaluateCompositingAfterLayout;
+    mutable bool m_needsToRecomputeCompositingRequirements;
+    mutable bool m_needsToUpdateLayerTreeGeometry;
 
     bool m_compositing;
     bool m_compositingLayersNeedRebuild;