Introduce PaintingContext, which will be used for custom painting

sky/engine/core/painting/PaintingContext.idl

Index: sky/engine/core/painting/PaintingContext.idl
diff --git a/sky/engine/core/painting/PaintingContext.idl b/sky/engine/core/painting/PaintingContext.idl
new file mode 100644
index 0000000000000000000000000000000000000000..58685006948d3c04391a01e5088e8d98ac1be228
--- /dev/null
+++ b/sky/engine/core/painting/PaintingContext.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.
+
+interface PaintingContext {
+  readonly attribute double height;
+  readonly attribute double width;
+
+  void drawCircle(double x, double y, double radius, Paint paint);
+
+  void commit();
+};