Destroy observers eagerly when StopTracking is called.

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 509 matching lines @@
 void PageLoadTracker::OnLoadedSubresource(bool was_cached) {
   if (was_cached) {
     ++num_cache_requests_;
   } else {
     ++num_network_requests_;
   }
 }
 
 void PageLoadTracker::StopTracking() {
   did_stop_tracking_ = true;
+  observers_.clear();
 }
 
 void PageLoadTracker::AddObserver(
     std::unique_ptr<PageLoadMetricsObserver> observer) {
   observers_.push_back(std::move(observer));
 }
 
 void PageLoadTracker::ClampBrowserTimestampIfInterProcessTimeTickSkew(
     base::TimeTicks* event_time) {
   DCHECK(event_time != nullptr);
@@ skipping 526 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