Make having an existing ancestor overflow layer when adding a PaintLayer fatal.

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

Index: third_party/WebKit/Source/core/paint/PaintLayer.cpp
diff --git a/third_party/WebKit/Source/core/paint/PaintLayer.cpp b/third_party/WebKit/Source/core/paint/PaintLayer.cpp
index 62a1ce16e876f295d51ecba6a033efc97d0e7185..77b9cddafecb866ce990f62ccc2089e1fa3b3102 100644
--- a/third_party/WebKit/Source/core/paint/PaintLayer.cpp
+++ b/third_party/WebKit/Source/core/paint/PaintLayer.cpp
@@ -1229,7 +1229,7 @@ void PaintLayer::addChild(PaintLayer* child, PaintLayer* beforeChild) {
 
   // The ancestor overflow layer is calculated during compositing inputs update
   // and should not be set yet.
-  DCHECK(!child->ancestorOverflowLayer());
+  CHECK(!child->ancestorOverflowLayer());
 
   setNeedsCompositingInputsUpdate();