Make PageLoadMetricsObserver::OnStart return ObservePolicy

chrome/browser/page_load_metrics/metrics_web_contents_observer.cc

 // 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.
 
 #include "chrome/browser/page_load_metrics/metrics_web_contents_observer.h"
 
 #include <algorithm>
 #include <ostream>
 #include <string>
 #include <utility>
@@ skipping 307 matching lines @@
   if (embedder_interface_->IsPrerendering(
           navigation_handle->GetWebContents())) {
     DCHECK(!started_in_foreground_);
     // For the time being, we do not track prerenders. See crbug.com/648338 for
     // details.
     StopTracking();
     return;
   }
 
   embedder_interface_->RegisterObservers(this);
-  for (const auto& observer : observers_) {
-    observer->OnStart(navigation_handle, currently_committed_url,
-                      started_in_foreground_);
-  }
+  INVOKE_AND_PRUNE_OBSERVERS(observers_, OnStart, navigation_handle,
+                             currently_committed_url, started_in_foreground_);
 }
 
 PageLoadTracker::~PageLoadTracker() {
   if (app_entered_background_) {
     RecordAppBackgroundPageLoadCompleted(true);
   }
 
   if (did_stop_tracking_)
     return;
 
@@ skipping 752 matching lines @@
     content::NavigationHandle* navigation_handle) const {
   DCHECK(navigation_handle->IsInMainFrame());
   DCHECK(!navigation_handle->HasCommitted() ||
          !navigation_handle->IsSamePage());
 
   return BrowserPageTrackDecider(embedder_interface_.get(), web_contents(),
                                  navigation_handle).ShouldTrack();
 }
 
 }  // namespace page_load_metrics