Tick off some TODOs:

include/core/SkDrawPictureCallback.h

+/*
+ * Copyright 2014 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#ifndef SkDrawPictureCallback_DEFINED
+#define SkDrawPictureCallback_DEFINED
+
+/**
+ *  Subclasses of this can be passed to canvas.drawPicture(). During the drawing
+ *  of the picture, this callback will periodically be invoked. If its
+ *  abortDrawing() returns true, then picture playback will be interrupted.
+ *
+ *  The resulting drawing is undefined, as there is no guarantee how often the
+ *  callback will be invoked. If the abort happens inside some level of nested
+ *  calls to save(), restore will automatically be called to return the state
+ *  to the same level it was before the drawPicture call was made.
+ */
+class SK_API SkDrawPictureCallback {
+public:
+    SkDrawPictureCallback() {}
+    virtual ~SkDrawPictureCallback() {}
+
+    virtual bool abortDrawing() = 0;
+};
+
+#endif//SkDrawPictureCallback_DEFINED