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 f329b31ef3fa39b8bf27b9fa01deec779e2860b8..6baebf02bfb5a7794626ffa4a5af5af5d5137593 100644
--- a/third_party/WebKit/Source/core/paint/PaintPropertyTreeBuilder.cpp
+++ b/third_party/WebKit/Source/core/paint/PaintPropertyTreeBuilder.cpp
@@ -319,6 +319,18 @@ 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;
 }