Fix a UBSanVptr bot failure in MediaNotificationService

Sample failure:
https://ci.chromium.org/p/chromium/builders/ci/UBSanVptr%20Linux/51514

Bug: 1137496
Change-Id: I462398f4d452cf6755fa40c72cbbfdacbbeaac34
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2585995Reviewed-by: Tommy Steimel <steimel@chromium.org>
Commit-Queue: Takumi Fujimoto <takumif@chromium.org>
Cr-Commit-Position: refs/heads/master@{#836262}

chrome/browser/ui/global_media_controls/media_notification_service.cc

@@ -692,10 +692,12 @@ void MediaNotificationService::SetDialogDelegate(
 
   std::list<std::string> sorted_session_ids;
   for (const std::string& id : active_controllable_session_ids_) {
-    if (sessions_.find(id)->second.IsPlaying())
+    auto session_it = sessions_.find(id);
+    if (session_it != sessions_.end() && session_it->second.IsPlaying()) {
       sorted_session_ids.push_front(id);
-    else
+    } else {
       sorted_session_ids.push_back(id);
+    }
   }
 
   for (const std::string& id : sorted_session_ids) {