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

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

 /*
  * Copyright (C) 2006, 2007, 2008, 2009, 2010, 2011, 2012 Apple Inc. All rights
  * reserved.
  *
  * Portions are Copyright (C) 1998 Netscape Communications Corporation.
  *
  * Other contributors:
  *   Robert O'Callahan <roc+@cs.cmu.edu>
  *   David Baron <dbaron@fas.harvard.edu>
  *   Christian Biesinger <cbiesinger@web.de>
@@ skipping 1211 matching lines @@
     child->setNextSibling(beforeChild);
     DCHECK(beforeChild != child);
   } else {
     setLastChild(child);
   }
 
   child->m_parent = this;
 
   // The ancestor overflow layer is calculated during compositing inputs update
   // and should not be set yet.
-  DCHECK(!child->ancestorOverflowLayer());
+  CHECK(!child->ancestorOverflowLayer());
 
   setNeedsCompositingInputsUpdate();
 
   if (!child->stackingNode()->isStacked() &&
       !layoutObject()->documentBeingDestroyed())
     compositor()->setNeedsCompositingUpdate(CompositingUpdateRebuildTree);
 
   if (child->stackingNode()->isStacked() || child->firstChild()) {
     // Dirty the z-order list in which we are contained. The
     // ancestorStackingContextNode() can be null in the case where we're
@@ skipping 1978 matching lines @@
 }
 
 void showLayerTree(const blink::LayoutObject* layoutObject) {
   if (!layoutObject) {
     LOG(INFO) << "Cannot showLayerTree. Root is (nil)";
     return;
   }
   showLayerTree(layoutObject->enclosingLayer());
 }
 #endif