Remove the HasOwnBackingButPaintsIntoAncestor compositing state.

Source/core/paint/BoxPainter.cpp

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "config.h"
 #include "core/paint/BoxPainter.h"
 
 #include "core/HTMLNames.h"
 #include "core/frame/Settings.h"
 #include "core/html/HTMLFrameOwnerElement.h"
@@ skipping 529 matching lines @@
 }
 
 void BoxPainter::paintClippingMask(PaintInfo& paintInfo, const LayoutPoint& paintOffset)
 {
     if (!paintInfo.shouldPaintWithinRoot(&m_renderBox) || m_renderBox.style()->visibility() != VISIBLE || paintInfo.phase != PaintPhaseClippingMask)
         return;
 
     if (!m_renderBox.layer() || m_renderBox.layer()->compositingState() != PaintsIntoOwnBacking)
         return;
 
-    // We should never have this state in this function. A layer with a mask
-    // should have always created its own backing if it became composited.
-    ASSERT(m_renderBox.layer()->compositingState() != HasOwnBackingButPaintsIntoAncestor);
-
     LayoutRect paintRect = LayoutRect(paintOffset, m_renderBox.size());
     paintInfo.context->fillRect(pixelSnappedIntRect(paintRect), Color::black);
 }
 
 void BoxPainter::paintRootBackgroundColor(RenderObject& obj, const PaintInfo& paintInfo, const LayoutRect& rect, const Color& bgColor)
 {
     GraphicsContext* context = paintInfo.context;
     if (rect.isEmpty())
         return;
 
@@ skipping 1584 matching lines @@
 
     FloatPoint secondQuad[4];
     secondQuad[0] = quad[0];
     secondQuad[1] = FloatPoint(quad[0].x() - r1 * cx, quad[0].y() - r1 * cy);
     secondQuad[2] = quad[2];
     secondQuad[3] = quad[3];
     graphicsContext->clipPolygon(4, secondQuad, !secondEdgeMatches);
 }
 
 } // namespace blink