Bug 1312216 - Changed delete this._openedURL and set it as null instead. There was a strict warning. r=Gijs

MozReview-Commit-ID: 3s3cojJVXcE

--HG--
extra : rebase_source : e9b3895988c92c1c3f6a102583d5b48cc8e1e9d7

browser/components/preferences/in-content/subdialogs.js

@@ -112,7 +112,7 @@ var gSubDialog = {
         if (this._frame.contentWindow.location.href == "about:blank") {
           this._frame.removeEventListener("load", onBlankLoad);
           // We're now officially done closing, so update the state to reflect that.
-          delete this._openedURL;
+          this._openedURL = null;
           this._isClosing = false;
           this._resolveClosePromise();
         }