Factor PageLoadTracker into its own header and impl files.

chrome/browser/page_load_metrics/metrics_web_contents_observer_unittest.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 "chrome/browser/page_load_metrics/metrics_web_contents_observer.h"
 
 #include <memory>
 #include <vector>
 
 #include "base/macros.h"
 #include "base/memory/ptr_util.h"
 #include "base/process/kill.h"
 #include "base/test/histogram_tester.h"
 #include "base/time/time.h"
 #include "chrome/browser/page_load_metrics/metrics_navigation_throttle.h"
+#include "chrome/browser/page_load_metrics/page_load_metrics_embedder_interface.h"
 #include "chrome/browser/page_load_metrics/page_load_metrics_observer.h"
+#include "chrome/browser/page_load_metrics/page_load_tracker.h"
 #include "chrome/common/page_load_metrics/page_load_metrics_messages.h"
 #include "chrome/test/base/chrome_render_view_host_test_harness.h"
 #include "content/public/browser/navigation_handle.h"
 #include "content/public/browser/render_frame_host.h"
 #include "content/public/test/test_renderer_host.h"
 #include "content/public/test/web_contents_tester.h"
 #include "testing/gtest/include/gtest/gtest.h"
 #include "url/gurl.h"
 
 namespace page_load_metrics {
@@ skipping 581 matching lines @@
   web_contents_tester->NavigateAndCommit(GURL(kDefaultTestUrl2));
   ASSERT_EQ(std::vector<GURL>({GURL(kDefaultTestUrl)}),
             completed_filtered_urls());
 
   web_contents_tester->NavigateAndCommit(GURL(kDefaultTestUrl));
   ASSERT_EQ(std::vector<GURL>({GURL(kDefaultTestUrl), GURL(kDefaultTestUrl2)}),
             completed_filtered_urls());
 }
 
 }  // namespace page_load_metrics