LongTaskTiming: add idl, Experiment flag, PerformanceEntry and basic implementation

third_party/WebKit/Source/core/timing/Performance.idl

 /*
  * Copyright (C) 2010 Google Inc. All rights reserved.
  * Copyright (C) 2012 Intel 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:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
@@ skipping 58 matching lines @@
 
     [MeasureAs=UnprefixedUserTiming, RaisesException] void measure(DOMString measureName, optional DOMString startMark = null, optional DOMString endMark = null);
     [MeasureAs=UnprefixedUserTiming] void clearMeasures(optional DOMString measureName = null);
 
     // Frame Timing
     // https://w3c.github.io/frame-timing/#extensions-performance-interface
     [RuntimeEnabled=FrameTimingSupport, MeasureAs=PerformanceFrameTiming] void clearFrameTimings();
     [RuntimeEnabled=FrameTimingSupport, MeasureAs=PerformanceFrameTiming] void setFrameTimingBufferSize(unsigned long maxSize);
     [RuntimeEnabled=FrameTimingSupport] attribute EventHandler onframetimingbufferfull;
 
+    // Long Task Timing
+    [RuntimeEnabled=LongTaskObserver] void clearLongTaskTimings();
+    [RuntimeEnabled=LongTaskObserver] void setLongTaskTimingBufferSize(unsigned long maxSize);
+    [RuntimeEnabled=LongTaskObserver] attribute EventHandler onlongtasktimingbufferfull;
+
     // TODO(foolip): There is no spec for the Memory Info API, see blink-dev:
     // https://groups.google.com/a/chromium.org/d/msg/blink-dev/g5YRCGpC9vs/b4OJz71NmPwJ
     [Measure] readonly attribute MemoryInfo memory;
 };