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 a3f98099ac75f0678239c3b733aa136cb8f4d552..7dc58b362364b676f5d54c4f696d1fcc8686eef6 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::setTransferMode(SkXfermode::Mode transfer_mode)
 {
     m_paint.setXfermodeMode(transfer_mode);