Add UMA to track time between preload discovery and reference

third_party/WebKit/Source/core/html/parser/HTMLResourcePreloader.cpp

 /*
  * Copyright (C) 2013 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
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 57 matching lines @@
     // TODO(yoichio): Should preload if document is imported.
     if (!m_document->loader())
         return;
     FetchRequest request = preload->resourceRequest(m_document);
     // TODO(dgozman): This check should go to HTMLPreloadScanner, but this requires
     // making Document::completeURLWithOverride logic to be statically accessible.
     if (request.url().protocolIsData())
         return;
     if (preload->resourceType() == Resource::Script || preload->resourceType() == Resource::CSSStyleSheet || preload->resourceType() == Resource::ImportResource)
         request.setCharset(preload->charset().isEmpty() ? m_document->characterSet().getString() : preload->charset());
-    request.setForPreload(true);
+    request.setForPreload(true, preload->discoveryTime());
     int duration = static_cast<int>(1000 * (monotonicallyIncreasingTime() - preload->discoveryTime()));
     DEFINE_STATIC_LOCAL(CustomCountHistogram, preloadDelayHistogram, ("WebCore.PreloadDelayMs", 0, 2000, 20));
     preloadDelayHistogram.count(duration);
     m_document->loader()->startPreload(preload->resourceType(), request);
 }
 
 } // namespace blink