Add support for linear gradients, implemented as skia shaders.

sky/engine/core/painting/Gradient.idl

Index: sky/engine/core/painting/Gradient.idl
diff --git a/sky/engine/core/painting/Gradient.idl b/sky/engine/core/painting/Gradient.idl
new file mode 100644
index 0000000000000000000000000000000000000000..aeea5b71d89f1e1faa939a9a9e1c866fbfd92817
--- /dev/null
+++ b/sky/engine/core/painting/Gradient.idl
@@ -0,0 +1,10 @@
+// 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.
+
+[
+    ImplementedAs=CanvasGradient,
+    CustomDart,
+    Constructor(unsigned long gradientType, Point[] endPoints, Color[] colors, float[] colorStops)
+] interface Gradient : Shader {
+};