[NoStatePrefetch] Fix the PrefetchAge histogram to match histograms.xml

chrome/browser/prerender/prerender_nostate_prefetch_browsertest.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "base/command_line.h"
 #include "base/strings/string16.h"
 #include "base/strings/string_split.h"
 #include "base/strings/utf_string_conversions.h"
 #include "base/task_scheduler/post_task.h"
 #include "chrome/browser/history/history_service_factory.h"
@@ skipping 160 matching lines @@
       test_utils::FirstContentfulPaintManagerWaiter::Create(
           GetPrerenderManager());
   ui_test_utils::NavigateToURL(
       current_browser(), src_server()->GetURL(MakeAbsolute(kPrefetchPage)));
   fcp_waiter->Wait();
 
   histogram_tester().ExpectTotalCount(
       "Prerender.websame_PrefetchTTFCP.Warm.Cacheable.Visible", 1);
   histogram_tester().ExpectTotalCount(
       "Prerender.websame_NoStatePrefetchResponseTypes", 2);
+  histogram_tester().ExpectTotalCount("Prerender.websame_PrefetchAge", 1);
 }
 
 // Checks the prefetch of an img tag.
 IN_PROC_BROWSER_TEST_F(NoStatePrefetchBrowserTest, PrefetchImage) {
   RequestCounter image_counter;
   CountRequestFor(kPrefetchJpeg, &image_counter);
   base::StringPairs replacement_text;
   replacement_text.push_back(
       std::make_pair("REPLACE_WITH_IMAGE_URL", MakeAbsolute(kPrefetchJpeg)));
   std::string main_page_path;
@@ skipping 405 matching lines @@
   // The SW intercepts kPrefetchPage and replaces it with a body that contains
   // an <img> tage for kPrefetchPng. This verifies that the SW ran correctly by
   // observing the fetch of the image.
   RequestCounter image_counter;
   CountRequestFor(kPrefetchPng, &image_counter);
   PrefetchFromFile(kPrefetchPage, FINAL_STATUS_NOSTATE_PREFETCH_FINISHED);
   image_counter.WaitForCount(1);
 }
 
 }  // namespace prerender