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

Source/devtools/front_end/inspector.css

 /*
  * Copyright (C) 2006, 2007, 2008 Apple Inc.  All rights reserved.
  * Copyright (C) 2009 Anthony Ricaud <rik@webkit.org>
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  *
  * 1.  Redistributions of source code must retain the above copyright
  *     notice, this list of conditions and the following disclaimer.
@@ skipping 74 matching lines @@
 }
 
 .hidden {
     display: none !important;
 }
 
 .white-background {
     background-color: white;
 }
 
+.overflow-hidden {
+    overflow: hidden;
+}
+
 .nowrap,
 .nowrap-below,
 .nowrap-below div,
 .nowrap-below span {
     white-space: nowrap !important;
 }
 
 .toolbar-background {
     background-image: linear-gradient(to bottom, rgb(191, 191, 191), rgb(151, 151, 151));
     padding-left: 1px;
@@ skipping 2782 matching lines @@
 }
 
 select.drop-down-menu {
     border: none;
     -webkit-appearance: none;
 }
 
 .viewport-control-gap-element {
     color: transparent;
 }