[DevTools] Move user agent override to responsive design toolbar.

Source/devtools/front_end/overrides.css

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 211 matching lines @@
     font: inherit;
     border-width: 1px;
 }
 
 .overrides-view input[type=text] {
     margin: 0;
     min-height: 2em;
     padding: 3px;
 }
 
+.overrides-view input[readonly] {
+    background-color: rgb(235, 235, 228);
+}
+
 .overrides-view .overrides-footer {
     flex: none;
     padding: 0 0 1px 3px;
     border-top: 1px solid rgb(203,203,203);
     background-color: rgb(236,236,236);
 }
 
 .overrides-view .overrides-footer::before {
     background-image: url(Images/statusbarButtonGlyphs.png);
     background-size: 320px 144px;
     width: 10px;
     height: 10px;
     content: "";
     position: relative;
     top: 2px;
     background-position: -202px -107px;
     float: left;
     margin-right: 4px;
 }
 
 @media (-webkit-min-device-pixel-ratio: 1.5) {
 .overrides-view .overrides-footer::before {
     background-image: url(Images/statusbarButtonGlyphs_2x.png);
 }
 } /* media */
 
 .overrides-view .overrides-footer .warning-icon-small {
     margin-right: 3px;
 }