Add UMA to track time between preload discovery and first bytes of response

third_party/WebKit/Source/core/fetch/Resource.cpp

 /*
     Copyright (C) 1998 Lars Knoll (knoll@mpi-hd.mpg.de)
     Copyright (C) 2001 Dirk Mueller (mueller@kde.org)
     Copyright (C) 2002 Waldo Bastian (bastian@kde.org)
     Copyright (C) 2006 Samuel Weinig (sam.weinig@gmail.com)
     Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011 Apple Inc. All rights reserved.
 
     This library is free software; you can redistribute it and/or
     modify it under the terms of the GNU Library General Public
     License as published by the Free Software Foundation; either
@@ skipping 564 matching lines @@
     if (!memoryCache()->contains(this) || hasClientsOrObservers() || !m_revalidatingRequest.isNull() || !m_loadFinishTime || !isSafeToUnlock())
         return false;
 
     m_data->unlock();
     return true;
 }
 
 void Resource::responseReceived(const ResourceResponse& response, PassOwnPtr<WebDataConsumerHandle>)
 {
     m_responseTimestamp = currentTime();
+    if (m_preloadDiscoveryTime) {
+        int timeSinceDiscovery = static_cast<int>(1000 * (monotonicallyIncreasingTime() - m_preloadDiscoveryTime));
+        DEFINE_STATIC_LOCAL(CustomCountHistogram, preloadDiscoveryToFirstByteHistogram, ("PreloadScanner.TTFB", 0, 10000, 50));
+        preloadDiscoveryToFirstByteHistogram.count(timeSinceDiscovery);
+    }
 
     if (!m_revalidatingRequest.isNull()) {
         if (response.httpStatusCode() == 304) {
             revalidationSucceeded(response);
             return;
         }
         revalidationFailed();
     }
     setResponse(response);
     String encoding = response.textEncodingName();
@@ skipping 534 matching lines @@
     case Resource::Media:
         return "Media";
     case Resource::Manifest:
         return "Manifest";
     }
     ASSERT_NOT_REACHED();
     return "Unknown";
 }
 
 } // namespace blink