Add support for linear gradients, implemented as skia shaders.

sky/engine/core/painting/Paint.h

 // 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.
 
 #ifndef SKY_ENGINE_CORE_PAINTING_PAINT_H_
 #define SKY_ENGINE_CORE_PAINTING_PAINT_H_
 
 #include "sky/engine/core/painting/CanvasColor.h"
 #include "sky/engine/core/painting/TransferMode.h"
 #include "sky/engine/tonic/dart_wrappable.h"
 #include "sky/engine/wtf/PassRefPtr.h"
 #include "sky/engine/wtf/RefCounted.h"
 #include "third_party/skia/include/core/SkPaint.h"
 
 namespace blink {
 
 class DrawLooper;
 class ColorFilter;
 class MaskFilter;
+class Shader;
 
 class Paint : public RefCounted<Paint>, public DartWrappable {
     DEFINE_WRAPPERTYPEINFO();
 public:
     ~Paint() override;
     static PassRefPtr<Paint> create()
     {
         return adoptRef(new Paint);
     }
 
     bool isAntiAlias() const { return m_paint.isAntiAlias(); }
     void setIsAntiAlias(bool value) { m_paint.setAntiAlias(value); }
 
     SkColor color() const { return m_paint.getColor(); }
     void setColor(SkColor color) { m_paint.setColor(color); }
 
     void setARGB(unsigned a, unsigned r, unsigned g, unsigned b)
     {
         m_paint.setARGB(a, r, g, b);
     }
     void setDrawLooper(DrawLooper* looper);
     void setColorFilter(ColorFilter* filter);
     void setMaskFilter(MaskFilter* filter);
+    void setShader(Shader* shader);
 
     const SkPaint& paint() const { return m_paint; }
     void setPaint(const SkPaint& paint) { m_paint = paint; }
 
     void setTransferMode(SkXfermode::Mode transfer_mode);
 
 private:
     Paint();
 
     SkPaint m_paint;
 };
 
 } // namespace blink
 
 #endif  // SKY_ENGINE_CORE_PAINTING_PAINT_H_