BoxBorderPainter should use computed color alpha

Source/core/style/BorderEdge.h

 // 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.
 
 #ifndef BorderEdge_h
 #define BorderEdge_h
 
 #include "core/style/ComputedStyleConstants.h"
 #include "platform/graphics/Color.h"
 
 namespace blink {
 
 struct BorderEdge {
-    BorderEdge(int edgeWidth, const Color& edgeColor, EBorderStyle edgeStyle, bool edgeIsTransparent, bool edgeIsPresent = true);
+    BorderEdge(int edgeWidth, const Color& edgeColor, EBorderStyle edgeStyle, bool edgeIsPresent = true);
     BorderEdge();
 
     bool hasVisibleColorAndStyle() const;
     bool shouldRender() const;
     bool presentButInvisible() const;
     bool obscuresBackgroundEdge() const;
     bool obscuresBackground() const;
     int usedWidth() const;
 
     bool sharesColorWith(const BorderEdge& other) const;
 
     EBorderStyle borderStyle() const  { return static_cast<EBorderStyle>(style); }
 
     enum DoubleBorderStripe {
         DoubleBorderStripeOuter,
         DoubleBorderStripeInner
     };
 
     int getDoubleBorderStripeWidth(DoubleBorderStripe) const;
 
     int width;
     Color color;
-    bool isTransparent;
     bool isPresent;
 
 private:
     unsigned style: 4; // EBorderStyle
 };
 
 } // namespace blink
 
 #endif // BorderEdge_h