Sky: Add a DrawLooper interface to the painting API to be used for shadows.

sky/engine/core/painting/LayerDrawLooperBuilder.idl

Index: sky/engine/core/painting/LayerDrawLooperBuilder.idl
diff --git a/sky/engine/core/painting/LayerDrawLooperBuilder.idl b/sky/engine/core/painting/LayerDrawLooperBuilder.idl
new file mode 100644
index 0000000000000000000000000000000000000000..79afe755f4df89f9247b7cf11a1c2a5991ee0495
--- /dev/null
+++ b/sky/engine/core/painting/LayerDrawLooperBuilder.idl
@@ -0,0 +1,12 @@
+// 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.
+[
+    Constructor()
+] interface LayerDrawLooperBuilder {
+  // Returns a DrawLooper based on the construction operations, and resets the
+  // builder back to a default state.
+  DrawLooper build();
+
+  void addLayerOnTop(DrawLooperLayerInfo info, DrawLooperAddLayerCallback callback);
+};