Bug 1603512 - Set the doScroll class on the correct dialog element r=bgrins

Differential Revision: https://phabricator.services.mozilla.com/D56988

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

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

@@ -430,7 +430,7 @@ SubDialog.prototype = {
       frameMinHeight = maxHeight + "px";
       let contentPane =
         this._frame.contentDocument.querySelector(".contentPane") ||
-        this._frame.contentDocument.documentElement;
+        this._frame.contentDocument.querySelector("dialog");
       contentPane.classList.add("doScroll");
     }