Bug 1575954 - QI for nsIDocShell before setting watchedByDevtools, r=jlast.

Differential Revision: https://phabricator.services.mozilla.com/D44340

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

devtools/server/actors/replay/replay.js

@@ -258,6 +258,7 @@ Services.obs.addObserver(
   {
     observe(subject, topic, data) {
       assert(topic == "webnavigation-create");
+      subject.QueryInterface(Ci.nsIDocShell);
       subject.watchedByDevtools = true;
     },
   },