Bug 914138 - picked wrong platform to run test, rs=me

browser/components/customizableui/test/browser.ini

@@ -20,7 +20,7 @@ support-files =
 
 [browser_914138_widget_API_overflowable_toolbar.js]
 # Because of the specific widths, this test is fragile and won't necessarily pass on other platforms
-run-if = os == "win"
+run-if = os == "mac"
 
 [browser_914863_disabled_help_quit_buttons.js]
 [browser_918049_skipintoolbarset_dnd.js]