Allow mousewheel scrolling to work in print preview on HiDPI devices

chrome/browser/resources/print_preview/previewarea/margin_control.css

 /* Copyright (c) 2012 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. */
 
 #preview-area .margin-control {
   -webkit-transition: opacity 150ms linear;
   position: absolute;
 }
 
 #preview-area .margin-control.invisible {
@@ skipping 16 matching lines @@
 }
 
 #preview-area .margin-control.margin-control-disabled {
   cursor: default;
 }
 
 #preview-area .margin-control-line {
   border-color: rgb(64, 128, 250);
   border-style: dashed;
   border-width: 1px;
+  /* TODO(raymes): This is a hack needed to force the margins onto separate
+   * layers to work around crbug.com/455439. We can remove this when it's fixed.
+   */
+  will-change: transform;
 }
 
 #preview-area .margin-control-disabled .margin-control-line  {
   border-color: gray;
 }
 
 #preview-area .margin-control-top .margin-control-line,
 #preview-area .margin-control-bottom .margin-control-line {
   width: 100%;
 }
@@ skipping 33 matching lines @@
 
 #preview-area .margin-control-bottom .margin-control-textbox {
   bottom: 8px;
   right: 50%;
 }
 
 #preview-area .margin-control-left .margin-control-textbox {
   bottom: 50%;
   left: 8px;
 }