Add override for overridden functions under /base

This CL was uploaded by git cl split.

R=dcheng@chromium.org

Bug: 428099
Change-Id: I50af2e37e8f60b8ca3de001d19528db50ae983a8
Reviewed-on: https://chromium-review.googlesource.com/1032464
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#554424}

base/observer_list_threadsafe.h

@@ -186,7 +186,7 @@ class ObserverListThreadSafe : public internal::ObserverListThreadSafeBase {
     Callback<void(ObserverType*)> method;
   };
 
-  ~ObserverListThreadSafe() = default;
+  ~ObserverListThreadSafe() override = default;
 
   void NotifyWrapper(ObserverType* observer,
                      const NotificationData& notification) {