[DevTools] Switch from DIP to CSS pixels in device mode.

Source/devtools/front_end/responsiveDesignView.css

 /*
  * Copyright 2014 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.
  */
 
 .responsive-design {
     overflow: hidden;
     position: relative;
 }
@@ skipping 358 matching lines @@
 .responsive-design-warning > div {
     flex: none;
 }
 
 .responsive-design-ruler-glasspane {
     position: absolute;
     left: 0px;
     right: 0px;
     top: 0px;
     background-color: transparent;
+    overflow: hidden;
 }
 
 .responsive-design-toggle-media-inspector .glyph {
     background-color: rgb(180, 180, 180);
     -webkit-mask-position: -128px -48px;
 }
 
 .responsive-design-toolbar button.responsive-design-toggle-media-inspector.toggled-on .glyph:not(.shadow) {
     background-color: rgb(105, 194, 236) !important;
 }
@@ skipping 142 matching lines @@
 
 @-webkit-keyframes media-inspector-animation-highlight-1 {
     from { -webkit-filter: brightness(135%); }
     to { -webkit-filter: brightness(100%); }
 }
 
 @-webkit-keyframes media-inspector-animation-highlight-2 {
     from { -webkit-filter: brightness(135%); }
     to { -webkit-filter: brightness(100%); }
 }