Add support for linear gradients, implemented as skia shaders.

sky/engine/core/painting/CanvasGradient.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_CANVASGRADIENT_H_
+#define SKY_ENGINE_CORE_PAINTING_CANVASGRADIENT_H_
+
+#include "sky/engine/core/painting/CanvasColor.h"
+#include "sky/engine/core/painting/Point.h"
+#include "sky/engine/core/painting/Shader.h"
+#include "sky/engine/tonic/dart_wrappable.h"
+#include "third_party/skia/include/effects/SkGradientShader.h"
+
+namespace blink {
+
+class CanvasGradient : public Shader {
+    DEFINE_WRAPPERTYPEINFO();
+ public:
+  ~CanvasGradient() override;
+  static PassRefPtr<CanvasGradient> create(int type,
+                                           const Vector<Point>& end_points,
+                                           const Vector<SkColor>& colors,
+                                           const Vector<float>& color_stops);
+
+ private:
+  CanvasGradient(PassRefPtr<SkShader> shader);
+};
+
+} // namespace blink
+
+#endif  // SKY_ENGINE_CORE_PAINTING_CANVASGRADIENT_H_