Add APIs for dynamic display lists, using SkDrawable and SkPictureRecorder::endRecordingAsDrawable.

sky/engine/core/painting/Drawable.idl

Index: sky/engine/core/painting/Drawable.idl
diff --git a/sky/engine/core/painting/Shader.idl b/sky/engine/core/painting/Drawable.idl
similarity index 76%
copy from sky/engine/core/painting/Shader.idl
copy to sky/engine/core/painting/Drawable.idl
index d29440d5fce72be5227edfd890d32f2473494ae6..a02c65503c87e0528eef97abb3eb9a338ff6f81f 100644
--- a/sky/engine/core/painting/Shader.idl
+++ b/sky/engine/core/painting/Drawable.idl
@@ -1,6 +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 Shader {
+interface Drawable {
+  Picture newPictureSnapshot();
 };