[page_load_metrics] Add a NavigationThrottle for richer abort metrics

chrome/browser/page_load_metrics/observers/aborts_page_load_metrics_observer.h

 // Copyright 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.
 
 #ifndef CHROME_BROWSER_PAGE_LOAD_METRICS_OBSERVERS_ABORTS_PAGE_LOAD_METRICS_OBSERVER_H_
 #define CHROME_BROWSER_PAGE_LOAD_METRICS_OBSERVERS_ABORTS_PAGE_LOAD_METRICS_OBSERVER_H_
 
 #include "base/macros.h"
 #include "chrome/browser/page_load_metrics/page_load_metrics_observer.h"
 
 namespace internal {
 
 extern const char kHistogramAbortForwardBackBeforeCommit[];
 extern const char kHistogramAbortReloadBeforeCommit[];
 extern const char kHistogramAbortNewNavigationBeforeCommit[];
 extern const char kHistogramAbortStopBeforeCommit[];
 extern const char kHistogramAbortCloseBeforeCommit[];
 extern const char kHistogramAbortOtherBeforeCommit[];
-extern const char kHistogramAbortUnknownNavigationBeforeCommit[];
 extern const char kHistogramAbortForwardBackBeforePaint[];
 extern const char kHistogramAbortReloadBeforePaint[];
 extern const char kHistogramAbortNewNavigationBeforePaint[];
 extern const char kHistogramAbortStopBeforePaint[];
 extern const char kHistogramAbortCloseBeforePaint[];
 
 }  // namespace internal
 
 class AbortsPageLoadMetricsObserver
     : public page_load_metrics::PageLoadMetricsObserver {
  public:
   AbortsPageLoadMetricsObserver();
 
   // page_load_metrics::PageLoadMetricsObserver:
   void OnComplete(
       const page_load_metrics::PageLoadTiming& timing,
       const page_load_metrics::PageLoadExtraInfo& extra_info) override;
   void OnFailedProvisionalLoad(
       const page_load_metrics::FailedProvisionalLoadInfo& failed_load_info,
       const page_load_metrics::PageLoadExtraInfo& extra_info) override;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(AbortsPageLoadMetricsObserver);
 };
 
 #endif  // CHROME_BROWSER_PAGE_LOAD_METRICS_OBSERVERS_ABORTS_PAGE_LOAD_METRICS_OBSERVER_H_