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 9c479bd031707b3fcc9d2ddcbd3427a8a33eaa23..bb0f7e5942a9dab7c9c5cb7be3ef2e7f41443887 100644
--- a/Source/core/rendering/RenderLayerCompositor.h
+++ b/Source/core/rendering/RenderLayerCompositor.h
@@ -46,7 +46,8 @@ enum CompositingUpdateType {
     CompositingUpdateAfterStyleChange,
     CompositingUpdateAfterLayout,
     CompositingUpdateOnScroll,
-    CompositingUpdateOnCompositedScroll
+    CompositingUpdateOnCompositedScroll,
+    CompositingUpdateFinishAllDeferredWork
 };
 
 // RenderLayerCompositor manages the hierarchy of
@@ -91,7 +92,9 @@ public:
     void updateCompositingRequirementsState();
     void setNeedsUpdateCompositingRequirementsState() { m_needsUpdateCompositingRequirementsState = true; }
 
-    // Rebuild the tree of compositing layers
+    // Main entry point for a full update. As needed, this function will compute compositing requirements,
+    // rebuild the composited layer tree, and/or update all the properties assocaited with each layer of the
+    // composited layer tree.
     void updateCompositingLayers(CompositingUpdateType, RenderLayer* updateRoot = 0);
 
     // Update the compositing state of the given layer. Returns true if that state changed.
@@ -200,7 +203,7 @@ public:
     void resetTrackedRepaintRects();
     void setTracksRepaints(bool);
 
-    void setShouldReevaluateCompositingAfterLayout() { m_reevaluateCompositingAfterLayout = true; }
+    void setNeedsToRecomputeCompositingRequirements() { m_needsToRecomputeCompositingRequirements = true; }
 
     // Returns all reasons (direct, indirectly due to subtree, and indirectly due to overlap) that a layer should be composited.
     CompositingReasons reasonsForCompositing(const RenderLayer*) const;
@@ -238,6 +241,10 @@ private:
     void addToOverlapMap(OverlapMap&, RenderLayer*, IntRect& layerBounds, bool& boundsComputed);
     void addToOverlapMapRecursive(OverlapMap&, RenderLayer*, RenderLayer* ancestorLayer = 0);
 
+    // Forces an update for all frames of frame tree recursively. Used only when the mainFrame compositor is ready to
+    // finish all deferred work.
+    static void finishCompositingUpdateForFrameTree(Frame*);
+
     // Returns true if any layer's compositing changed
     void computeCompositingRequirements(RenderLayer* ancestorLayer, RenderLayer*, OverlapMap*, struct CompositingRecursionData&, bool& layersChanged, bool& descendantHas3DTransform, Vector<RenderLayer*>& unclippedDescendants);
 
@@ -317,9 +324,9 @@ private:
     int m_compositedLayerCount;
     bool m_showRepaintCounter;
 
-    // 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;
+    // FIXME: This should absolutely not be mutable.
+    mutable bool m_needsToRecomputeCompositingRequirements;
+    bool m_needsToUpdateLayerTreeGeometry;
 
     bool m_compositing;
     bool m_compositingLayersNeedRebuild;