Bug 1690879: Make test_bug869314.html pass on macOS with the non-native theme enabled. r=emilio

Depends on D104084

Differential Revision: https://phabricator.services.mozilla.com/D104091

layout/forms/test/test_bug869314.html

@@ -30,12 +30,13 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=869314
   </select>
 
   <script type="application/javascript">
-  var Cc = SpecialPowers.Cc;
-  var Ci = SpecialPowers.Ci;
-  var sysInfo = Cc["@mozilla.org/system-info;1"].getService(Ci.nsIPropertyBag2);
-  var osName = sysInfo.getProperty("name");
-  if (osName == "Darwin") { // Mac OS X.
-    // This test is for Mac only. See bug for more info.
+  let Cc = SpecialPowers.Cc;
+  let Ci = SpecialPowers.Ci;
+  let sysInfo = Cc["@mozilla.org/system-info;1"].getService(Ci.nsIPropertyBag2);
+  let osName = sysInfo.getProperty("name");
+  let isNNT = SpecialPowers.getBoolPref("widget.disable-native-theme-for-content");
+  if (osName == "Darwin" && !isNNT) { // Native styled macOS form controls.
+    // This test is for macOS with native styled form controls only. See bug for more info.
     ok(document.getElementById("selectbox1").clientWidth >
        document.getElementById("selectbox2").clientWidth,
        "Non-native styled combobox does not have enough space for a " +