Destroy observers eagerly when StopTracking is called.

chrome/browser/page_load_metrics/metrics_web_contents_observer.cc

Index: chrome/browser/page_load_metrics/metrics_web_contents_observer.cc
diff --git a/chrome/browser/page_load_metrics/metrics_web_contents_observer.cc b/chrome/browser/page_load_metrics/metrics_web_contents_observer.cc
index f02d21cb487b8b7fd403f6af53a95ad9858636f7..b3454624fef18d7fea059784ccc04c1bc812c671 100644
--- a/chrome/browser/page_load_metrics/metrics_web_contents_observer.cc
+++ b/chrome/browser/page_load_metrics/metrics_web_contents_observer.cc
@@ -527,6 +527,7 @@ void PageLoadTracker::OnLoadedSubresource(bool was_cached) {
 
 void PageLoadTracker::StopTracking() {
   did_stop_tracking_ = true;
+  observers_.clear();
 }
 
 void PageLoadTracker::AddObserver(