BoxBorderPainter should use computed color alpha

Source/core/paint/BoxBorderPainter.cpp

 // Copyright 2015 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/BoxBorderPainter.h"
 
 #include "core/paint/BoxPainter.h"
 #include "core/paint/PaintInfo.h"
 #include "core/style/BorderEdge.h"
@@ skipping 528 matching lines @@
 
         if (!edge.shouldRender()) {
             if (edge.presentButInvisible()) {
                 m_isUniformWidth = false;
                 m_isUniformColor = false;
             }
 
             continue;
         }
 
+        ASSERT(edge.color.alpha() > 0);
+
         m_visibleEdgeCount++;
         m_visibleEdgeSet |= edgeFlagForSide(static_cast<BoxSide>(i));
 
         m_hasAlpha |= edge.color.hasAlpha();
 
         if (m_visibleEdgeCount == 1) {
             m_firstVisibleEdge = i;
             continue;
         }
 
@@ skipping 628 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, secondMiter == SoftMiter);
     }
 }
 
 } // namespace blink