optimizing layout performance when only transform3d matrix changed by

Source/core/rendering/RenderObject.cpp

Index: Source/core/rendering/RenderObject.cpp
===================================================================
--- Source/core/rendering/RenderObject.cpp	(revision 169401)
+++ Source/core/rendering/RenderObject.cpp	(working copy)
@@ -1826,8 +1826,11 @@
                 diff = StyleDifferenceSimplifiedLayout;
             else if (diff < StyleDifferenceSimplifiedLayout)
                 diff = StyleDifferenceSimplifiedLayoutAndPositionedMovement;
-        } else if (diff < StyleDifferenceRecompositeLayer)
-            diff = StyleDifferenceRecompositeLayer;
+        } else if (diff < StyleDifferenceRecompositeLayer) {
+            if (diff <= StyleDifferenceRecompositeLayer && (contextSensitiveProperties & ContextSensitivePropertyTransform3D))
+                diff = StyleDifferenceMinRecompositeLayer;
+        }
+
     }
 
     // If opacity or filters changed, and the layer does not paint into its own separate backing, then we need to repaint (also
@@ -1836,7 +1839,7 @@
         if (!isText() && (!hasLayer() || !toRenderLayerModelObject(this)->layer()->hasDirectReasonsForCompositing()))
             diff = StyleDifferenceRepaintLayer;
         else if (diff < StyleDifferenceRecompositeLayer)
-            diff = StyleDifferenceRecompositeLayer;
+            diff = StyleDifferenceMinRecompositeLayer;
     }
 
     if ((contextSensitiveProperties & ContextSensitivePropertyFilter) && hasLayer() && diff <= StyleDifferenceRepaintLayer) {
@@ -1966,6 +1969,11 @@
         } else if (updatedDiff == StyleDifferenceSimplifiedLayout)
             setNeedsSimplifiedNormalFlowLayout();
     }
+    if (updatedDiff > StyleDifferenceMinRecompositeLayer) {
+        TRACE_EVENT0("webkit", "setCompleteCompositing");
+        // trigger complete compositing
+        document().setFastCompositing(false);
+    }
 
     if (updatedDiff == StyleDifferenceRepaintLayer || shouldRepaintForStyleDifference(updatedDiff)) {
         // Do a repaint with the new style now, e.g., for example if we go from