Add support for linear gradients, implemented as skia shaders.

sky/engine/core/painting/Paint.h

Index: sky/engine/core/painting/Paint.h
diff --git a/sky/engine/core/painting/Paint.h b/sky/engine/core/painting/Paint.h
index 71ac40573207b2a3fe8d5fd720c16070fd404c92..5b56ea541d4472480191001ea7a12c62c925e430 100644
--- a/sky/engine/core/painting/Paint.h
+++ b/sky/engine/core/painting/Paint.h
@@ -17,6 +17,7 @@ namespace blink {
 class DrawLooper;
 class ColorFilter;
 class MaskFilter;
+class Shader;
 
 class Paint : public RefCounted<Paint>, public DartWrappable {
     DEFINE_WRAPPERTYPEINFO();
@@ -40,6 +41,7 @@ public:
     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; }