Prevent tracking metrics for cases such as 204s and downloads.

chrome/browser/page_load_metrics/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_PAGE_LOAD_METRICS_OBSERVER_H_
 #define CHROME_BROWSER_PAGE_LOAD_METRICS_PAGE_LOAD_METRICS_OBSERVER_H_
 
 #include "base/macros.h"
 #include "base/optional.h"
 #include "chrome/common/page_load_metrics/page_load_timing.h"
@@ skipping 236 matching lines @@
   // collected over the course of the page load. Most observers should not need
   // to implement these callbacks, and should implement the On* timing callbacks
   // instead.
 
   // OnComplete is invoked for tracked page loads that committed, immediately
   // before the observer is deleted.
   virtual void OnComplete(const PageLoadTiming& timing,
                           const PageLoadExtraInfo& extra_info) {}
 
   // OnFailedProvisionalLoad is invoked for tracked page loads that did not
-  // commit, immediately before the observer is deleted. Note that provisional
-  // loads that result in downloads or 204s are aborted by the system, and are
-  // also included as failed provisional loads.
+  // commit, immediately before the observer is deleted.
   virtual void OnFailedProvisionalLoad(
       const FailedProvisionalLoadInfo& failed_provisional_load_info,
       const PageLoadExtraInfo& extra_info) {}
 };
 
 }  // namespace page_load_metrics
 
 #endif  // CHROME_BROWSER_PAGE_LOAD_METRICS_PAGE_LOAD_METRICS_OBSERVER_H_