Add support for linear gradients, implemented as skia shaders.

sky/engine/core/painting/Shader.idl

Index: sky/engine/core/painting/Shader.idl
diff --git a/sky/engine/core/painting/Picture.idl b/sky/engine/core/painting/Shader.idl
similarity index 89%
copy from sky/engine/core/painting/Picture.idl
copy to sky/engine/core/painting/Shader.idl
index 0f618fdef72cafeff11960cb00f1fb24a959a6b0..d29440d5fce72be5227edfd890d32f2473494ae6 100644
--- a/sky/engine/core/painting/Picture.idl
+++ b/sky/engine/core/painting/Shader.idl
@@ -1,5 +1,6 @@
 // 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.
-interface Picture {
+
+interface Shader {
 };