Bug 1501138 - Make sure that timeout iterator advances. r=peterv

Differential Revision: https://phabricator.services.mozilla.com/D11327

--HG--
extra : moz-landing-system : lando

dom/base/TimeoutManager.cpp

@@ -793,6 +793,7 @@ TimeoutManager::RunTimeout(const TimeStamp& aNow, const TimeStamp& aTargetDeadli
     for (RefPtr<Timeout> timeout = mTimeouts.GetFirst();
          timeout != nullptr;
          timeout = next) {
+      next = timeout->getNext();
       // We should only execute callbacks for the set of expired Timeout
       // objects we computed above.
       if (timeout->mFiringId != firingId) {