Add a basic custom painting facility to Sky

sky/engine/core/painting/PaintingCallback.h

Index: sky/engine/core/painting/PaintingCallback.h
diff --git a/sky/engine/core/painting/PaintingCallback.h b/sky/engine/core/painting/PaintingCallback.h
new file mode 100644
index 0000000000000000000000000000000000000000..7a5d77f5526682929aca890115fa449b07b30fa9
--- /dev/null
+++ b/sky/engine/core/painting/PaintingCallback.h
@@ -0,0 +1,19 @@
+// 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.
+
+#ifndef SKY_ENGINE_CORE_PAINTING_PAINTINGCALLBACK_H_
+#define SKY_ENGINE_CORE_PAINTING_PAINTINGCALLBACK_H_
+
+namespace blink {
+class PaintingContext;
+
+class PaintingCallback {
+public:
+    virtual ~PaintingCallback();
+    virtual void handleEvent(PaintingContext* context) = 0;
+};
+
+} // namespace blink
+
+#endif  // SKY_ENGINE_CORE_PAINTING_PAINTINGCALLBACK_H_