Polymer WebUI: Enable native CSS custom properties

ui/webui/resources/cr_elements/cr_search_field/cr_search_field.css

 /* Copyright 2015 The Chromium Authors. All rights reserved.
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file. */
 
 :host {
   -webkit-padding-end: 10px;
   box-sizing: border-box;
   display: flex;
   justify-content: flex-end;
 }
 
 [hidden] {
   display: none !important;
 }
 
 paper-input-container {
   margin-top: 2px;
   max-width: 200px;
   padding: 2px 0;
   width: 100%;
 }
 
 #searchTerm {
   --paper-input-container-color: rgb(192, 199, 205);
   --paper-input-container-focus-color: rgb(192, 199, 205);
   --paper-input-container-input: {
     color: inherit;
-    font-family: inherit;
     font-size: inherit;
   };
   --paper-input-container-input-color: rgb(192, 199, 205);
   color: rgb(192, 199, 205);
   z-index: 0;
 }
 
 #searchTerm input[type='search']::-webkit-search-decoration,
 #searchTerm input[type='search']::-webkit-search-cancel-button,
 #searchTerm input[type='search']::-webkit-search-results-button {
@@ skipping 27 matching lines @@
   position: absolute;
   right: 0;
   top: -4px;
   z-index: 1;
 }
 
 :host-context([dir='rtl']) #searchTerm paper-icon-button {
   left: 0;
   right: auto;
 }