Implement merging non-composited paint property nodes in the PACompositor.

third_party/WebKit/Source/core/paint/PaintPropertyTreeBuilder.cpp

Index: third_party/WebKit/Source/core/paint/PaintPropertyTreeBuilder.cpp
diff --git a/third_party/WebKit/Source/core/paint/PaintPropertyTreeBuilder.cpp b/third_party/WebKit/Source/core/paint/PaintPropertyTreeBuilder.cpp
index d566bce62457849e08dff4dd1d9c86918207e0e1..b57d92382a7e894d5d204eda94a4951664047564 100644
--- a/third_party/WebKit/Source/core/paint/PaintPropertyTreeBuilder.cpp
+++ b/third_party/WebKit/Source/core/paint/PaintPropertyTreeBuilder.cpp
@@ -318,6 +318,18 @@ static CompositingReasons compositingReasonsForTransform(
       !object.styleRef().subtreeWillChangeContents())
     compositingReasons |= CompositingReasonWillChangeCompositingHint;
 
+  if (object.isBoxModelObject()) {
+    const LayoutBoxModelObject* box = toLayoutBoxModelObject(&object);
+    if (box->hasLayer()) {
+      // TODO(chrishtr): move this to the descendant-dependent flags recursion
+      // PaintLayer::updateDescendantDependentFlags.
+      box->layer()->update3DTransformedDescendantStatus();
+
+      if (box->layer()->has3DTransformedDescendant())
+        compositingReasons |= CompositingReason3DTransform;
+    }
+  }
+
   return compositingReasons;
 }