Add the unprefixed appearance property.

less/close.less

@@ -32,5 +32,6 @@
     background: transparent;
     border: 0;
     -webkit-appearance: none;
+    appearance: none;
   }
 }

less/forms.less

@@ -56,6 +56,7 @@ input[type="search"] {
   // be added to `.form-control` as it's not specific enough. For details, see
   // https://github.com/twbs/bootstrap/issues/11586.
   -webkit-appearance: none;
+  appearance: none;
 }
 
 // Position radios and checkboxes better