Add UMA to track time between preload discovery and reference

third_party/WebKit/Source/core/loader/LinkLoader.cpp

 /*
  * 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:
  *
  *     * 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 278 matching lines @@
     ResourceRequest resourceRequest(document.completeURL(href));
     ResourceFetcher::determineRequestContext(resourceRequest, resourceType, false);
     FetchRequest linkRequest(resourceRequest, FetchInitiatorTypeNames::link, document.encodingName());
 
     linkRequest.setPriority(document.fetcher()->loadPriority(resourceType, linkRequest));
     if (crossOrigin != CrossOriginAttributeNotSet)
         linkRequest.setCrossOriginAccessControl(document.getSecurityOrigin(), crossOrigin);
     Settings* settings = document.settings();
     if (settings && settings->logPreload())
         document.addConsoleMessage(ConsoleMessage::create(OtherMessageSource, DebugMessageLevel, String("Preload triggered for " + href.host() + href.path())));
-    linkRequest.setForPreload(true);
+    linkRequest.setForPreload(true, monotonicallyIncreasingTime());
     linkRequest.setLinkPreload(true);
     return document.loader()->startPreload(resourceType, linkRequest);
 }
 
 void LinkLoader::loadLinksFromHeader(const String& headerValue, const KURL& baseURL, Document* document, const NetworkHintsInterface& networkHintsInterface,
     CanLoadResources canLoadResources, ViewportDescriptionWrapper* viewportDescriptionWrapper)
 {
     if (!document || headerValue.isEmpty())
         return;
     LinkHeaderSet headerSet(headerValue);
@@ skipping 83 matching lines @@
 DEFINE_TRACE(LinkLoader)
 {
     visitor->trace(m_client);
     visitor->trace(m_prerender);
     visitor->trace(m_linkPreloadResourceClient);
     ResourceOwner<Resource, ResourceClient>::trace(visitor);
     PrerenderClient::trace(visitor);
 }
 
 } // namespace blink