make Clear Private Data window app-modal on Mac (otherwise we get odd window positioning). b=309406 r=mano sr=mconnor

browser/base/content/sanitize.js

@@ -297,7 +297,11 @@ Sanitizer.showUI = function(aParentWindow)
 {
   var ww = Components.classes["@mozilla.org/embedcomp/window-watcher;1"]
                      .getService(Components.interfaces.nsIWindowWatcher);
+#ifdef XP_MACOSX
+  ww.openWindow(null, // make this an app-modal window on Mac
+#else
   ww.openWindow(aParentWindow,
+#endif
                 "chrome://browser/content/sanitize.xul",
                 "Sanitize",
                 "chrome,titlebar,centerscreen,modal",