HistoryBackend: Fix a crash in BroadcastNotification.

chrome/browser/history/history_backend.cc

Index: chrome/browser/history/history_backend.cc
diff --git a/chrome/browser/history/history_backend.cc b/chrome/browser/history/history_backend.cc
index 45f12ef5f54d8f21d8c748a4c79a32152f3a0a58..d16c6c67f6e474aa3a6bae2858f64660445cba2f 100644
--- a/chrome/browser/history/history_backend.cc
+++ b/chrome/browser/history/history_backend.cc
@@ -2072,8 +2072,10 @@ void HistoryBackend::ProcessDBTask(
 void HistoryBackend::BroadcastNotifications(
     int type,
     HistoryDetails* details_deleted) {
-  DCHECK(delegate_.get());
-  delegate_->BroadcastNotifications(type, details_deleted);
+  // |delegate_| may be NULL if |this| is in the process of closing (closed by
+  // HistoryService -> HistroyBackend::Closing().
+  if (delegate_.get())
+    delegate_->BroadcastNotifications(type, details_deleted);
 }
 
 // Deleting --------------------------------------------------------------------