Add CSSTiming to collect aggregate PLT-level stats about CSS.

third_party/WebKit/public/web/WebPerformance.h

 /*
  * Copyright (C) 2010 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:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 80 matching lines @@
   BLINK_EXPORT double firstContentfulPaint() const;
   BLINK_EXPORT double firstMeaningfulPaint() const;
   BLINK_EXPORT double parseStart() const;
   BLINK_EXPORT double parseStop() const;
   BLINK_EXPORT double parseBlockedOnScriptLoadDuration() const;
   BLINK_EXPORT double parseBlockedOnScriptLoadFromDocumentWriteDuration() const;
   BLINK_EXPORT double parseBlockedOnScriptExecutionDuration() const;
   BLINK_EXPORT double parseBlockedOnScriptExecutionFromDocumentWriteDuration()
       const;
 
+  // In microseconds (seconds are too coarse).

[Bryan McQuade, 2016/11/04 14:11:30]

are seconds really too coarse if you are using double precision? reading
https://en.wikipedia.org/wiki/Double-precision_floating-point_format it seems
like you have plenty of precision in a double.

[Charlie Harrison, 2016/11/04 16:05:19]

As discussed offline, I think this really has to do with microsecond clamping
done in PerformanceTiming. I'll try to fix it up.

+  BLINK_EXPORT double authorStyleSheetParseDurationBeforeFCP() const;
+
 #if BLINK_IMPLEMENTATION
   WebPerformance(Performance*);
   WebPerformance& operator=(Performance*);
 #endif
 
  private:
   WebPrivatePtr<Performance> m_private;
 };
 
 }  // namespace blink
 
 #endif