Add support for linear gradients, implemented as skia shaders.

sky/engine/core/painting/Shader.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_SHADER_H_
+#define SKY_ENGINE_CORE_PAINTING_SHADER_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/SkShader.h"
+
+namespace blink {
+
+class Shader : public RefCounted<Shader>, public DartWrappable {
+  DEFINE_WRAPPERTYPEINFO();
+ public:
+  ~Shader() override;
+
+  SkShader* shader() { return shader_.get(); }
+
+ protected:
+  Shader(PassRefPtr<SkShader> shader);
+
+ private:
+  RefPtr<SkShader> shader_;
+};
+
+} // namespace blink
+
+#endif  // SKY_ENGINE_CORE_PAINTING_SHADER_H_