Add support for linear gradients, implemented as skia shaders.

sky/engine/core/painting/Paint.cpp

Index: sky/engine/core/painting/Paint.cpp
diff --git a/sky/engine/core/painting/Paint.cpp b/sky/engine/core/painting/Paint.cpp
index c11fa674d96c24e0b00b536af867af4103777dfa..8728d013cfb70647074e5504449988f9cc90fa06 100644
--- a/sky/engine/core/painting/Paint.cpp
+++ b/sky/engine/core/painting/Paint.cpp
@@ -8,6 +8,7 @@
 #include "sky/engine/core/painting/ColorFilter.h"
 #include "sky/engine/core/painting/DrawLooper.h"
 #include "sky/engine/core/painting/MaskFilter.h"
+#include "sky/engine/core/painting/Shader.h"
 
 namespace blink {
 
@@ -37,6 +38,12 @@ void Paint::setMaskFilter(MaskFilter* filter)
     m_paint.setMaskFilter(filter->filter());
 }
 
+void Paint::setShader(Shader* shader)
+{
+    ASSERT(shader);
+    m_paint.setShader(shader->shader());
+}
+
 void Paint::setStyle(SkPaint::Style style)
 {
     m_paint.setStyle(style);