PrintPreview: Make ctrl-shift-p start the native print flow.

chrome/browser/resources/print_preview/print_preview.js

 // Copyright (c) 2011 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.
 
 // require: cr/ui/print_preview_cloud.js
 
 var localStrings = new LocalStrings();
 
 // If useCloudPrint is true we attempt to connect to cloud print
 // and populate the list of printers with cloud print printers.
@@ skipping 122 matching lines @@
   var els = $('sidebar').querySelectorAll('input, button, select');
   for (var i = 0; i < els.length; i++)
     els[i].disabled = true;
 }
 
 /**
  * Disables the controls in the sidebar, shows the throbber and instructs the
  * backend to open the native print dialog.
  */
 function onSystemDialogLinkClicked() {
+  if (showingSystemDialog)
+    return;
   showingSystemDialog = true;
   disableInputElementsInSidebar();
   $('system-dialog-throbber').classList.remove('hidden');
   chrome.send('showSystemDialog');
 }
 
 /**
  * Similar to onSystemDialogLinkClicked(), but specific to the
  * 'Launch native print dialog' UI.
  */
@@ skipping 827 matching lines @@
 <include src="print_preview_animations.js"/>
 <include src="print_preview_cloud.js"/>
 <include src="print_preview_utils.js"/>
 <include src="print_header.js"/>
 <include src="page_settings.js"/>
 <include src="copies_settings.js"/>
 <include src="header_footer_settings.js"/>
 <include src="layout_settings.js"/>
 <include src="color_settings.js"/>
 <include src="margin_settings.js"/>