Factor PageLoadTracker into its own header and impl files.

chrome/browser/page_load_metrics/page_load_metrics_browsertest.cc

 // Copyright (c) 2015 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/files/scoped_temp_dir.h"
 #include "base/macros.h"
 #include "base/test/histogram_tester.h"
 #include "base/time/time.h"
 #include "chrome/browser/page_load_metrics/metrics_web_contents_observer.h"
 #include "chrome/browser/page_load_metrics/observers/aborts_page_load_metrics_observer.h"
 #include "chrome/browser/page_load_metrics/observers/core_page_load_metrics_observer.h"
 #include "chrome/browser/page_load_metrics/observers/document_write_page_load_metrics_observer.h"
 #include "chrome/browser/page_load_metrics/observers/no_state_prefetch_page_load_metrics_observer.h"
+#include "chrome/browser/page_load_metrics/page_load_tracker.h"
 #include "chrome/browser/prerender/prerender_histograms.h"
 #include "chrome/browser/prerender/prerender_origin.h"
 #include "chrome/browser/profiles/profile.h"
 #include "chrome/browser/ui/browser.h"
 #include "chrome/browser/ui/browser_navigator_params.h"
 #include "chrome/browser/ui/tabs/tab_strip_model.h"
 #include "chrome/common/pref_names.h"
 #include "chrome/common/url_constants.h"
 #include "chrome/test/base/in_process_browser_test.h"
 #include "chrome/test/base/ui_test_utils.h"
@@ skipping 504 matching lines @@
 
   histogram_tester_.ExpectTotalCount(
       prerender::PrerenderHistograms::GetFirstContentfulPaintHistogramName(
           prerender::ORIGIN_NONE, false, true, base::TimeDelta()),
       1);
   histogram_tester_.ExpectTotalCount(
       prerender::PrerenderHistograms::GetFirstContentfulPaintHistogramName(
           prerender::ORIGIN_NONE, false, false, base::TimeDelta()),
       0);
 }