Return the enclosing composited PaintLayer rather than containingBlock for floats.

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 900 matching lines @@
   TransformState transformState(TransformState::ApplyTransformDirection,
                                 FloatPoint());
   layoutObject()->mapLocalToAncestor(
       transformAncestor() ? transformAncestor()->layoutObject() : nullptr,
       transformState, 0);
   transformState.flatten();
   return LayoutPoint(transformState.lastPlanarPoint());
 }
 
 PaintLayer* PaintLayer::compositingContainer() const {
-  // Floats have special paintinng order. Also, the container does not need
-  // to be a stacking context, because floats are not stacked.
+  // Floats have special painting order, which has complicated semantics.
+  // See the comments around FloatObject::setShouldPaint.
   if (m_layoutObject->isFloating() && m_layoutObject->parent() &&
       !m_layoutObject->parent()->isLayoutBlockFlow())
     return m_layoutObject->containingBlock()->enclosingLayer();
   if (!stackingNode()->isStacked())
     return parent();
   if (PaintLayerStackingNode* ancestorStackingNode =
           stackingNode()->ancestorStackingContextNode())
     return ancestorStackingNode->layer();
   return nullptr;
 }
@@ skipping 2295 matching lines @@
 }
 
 void showLayerTree(const blink::LayoutObject* layoutObject) {
   if (!layoutObject) {
     LOG(INFO) << "Cannot showLayerTree. Root is (nil)";
     return;
   }
   showLayerTree(layoutObject->enclosingLayer());
 }
 #endif