Let Dart code running in Sky add events to the trace timeline

sky/engine/core/frame/Window.idl

 /*
  * Copyright (C) 2006, 2007, 2008, 2009 Apple Inc. All rights reserved.
  * Copyright (C) 2011 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
@@ skipping 54 matching lines @@
     [Replaceable] readonly attribute double devicePixelRatio;
 
     // This is the interface orientation in degrees. Some examples are:
     //  0 is straight up; -90 is when the device is rotated 90 clockwise;
     //  90 is when rotated counter clockwise.
     readonly attribute long orientation;
 
     long requestAnimationFrame(RequestAnimationFrameCallback callback);
     void cancelAnimationFrame(long id);
 
+    readonly attribute Tracing tracing;
+
     // [Replaceable] readonly attribute CSS CSS;
 };
 
 Window implements WindowBase64;
 Window implements WindowTimers;