Make it possible to custom-paint without an Element.

sky/engine/core/painting/PictureRecorder.idl

Index: sky/engine/core/painting/PictureRecorder.idl
diff --git a/shell/test/pingable.mojom b/sky/engine/core/painting/PictureRecorder.idl
similarity index 60%
copy from shell/test/pingable.mojom
copy to sky/engine/core/painting/PictureRecorder.idl
index f968344b57b088895f3d9571743e55b58dae8a08..b1ea18510b980135bc66694d42e9bd76c926f779 100644
--- a/shell/test/pingable.mojom
+++ b/sky/engine/core/painting/PictureRecorder.idl
@@ -1,7 +1,8 @@
 // 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 Pingable {
-  Ping(string message) => (string app_url, string connection_url, string message);
+[
+    Constructor(double width, double height)
+] interface PictureRecorder : Canvas {
+  Picture endRecording();
 };