Allow layers that need squashing to draw into the squashing layer in some cases.

Source/core/rendering/RenderLayer.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>
  *   Randall Jesup <rjesup@wgate.com>
@@ skipping 1116 matching lines @@
         return const_cast<RenderLayer*>(this);
 
     for (const RenderLayer* curr = compositingContainer(); curr; curr = curr->compositingContainer()) {
         if (curr->compositingState() != NotComposited && curr->compositingState() != PaintsIntoGroupedBacking)
             return const_cast<RenderLayer*>(curr);
     }
 
     return 0;
 }
 
+RenderLayer* RenderLayer::containingGroupedLayer() const
+{
+    ASSERT(isAllowedToQueryCompositingState());
+
+    for (const RenderLayer* curr = this; curr; curr = curr->parent()) {

[abarth-chromium, 2014/05/05 21:37:40]

Is this the right parent chain to walk?  I would have expected you to walk the
parents in the Stacking tree...

[chrishtr, 2014/05/05 21:58:51]

Right this is I think the issue with this CL.

+        switch (curr->compositingState()) {
+        case NotComposited:
+            continue;
+        case PaintsIntoGroupedBacking:
+                return const_cast<RenderLayer*>(curr);
+            break;
+        default:
+            return 0;
+            break;
+        }
+    }
+
+    return 0;
+}
+
 RenderLayer* RenderLayer::enclosingCompositingLayerForRepaint(IncludeSelfOrNot includeSelf) const
 {
     ASSERT(isAllowedToQueryCompositingState());
 
     if ((includeSelf == IncludeSelf) && (compositingState() == PaintsIntoOwnBacking || compositingState() == PaintsIntoGroupedBacking))
         return const_cast<RenderLayer*>(this);
 
     for (const RenderLayer* curr = compositingContainer(); curr; curr = curr->compositingContainer()) {
         if (curr->compositingState() == PaintsIntoOwnBacking || curr->compositingState() == PaintsIntoGroupedBacking)
             return const_cast<RenderLayer*>(curr);
@@ skipping 2874 matching lines @@
     }
 }
 
 void showLayerTree(const WebCore::RenderObject* renderer)
 {
     if (!renderer)
         return;
     showLayerTree(renderer->enclosingLayer());
 }
 #endif