Prevent the PDF iframe from being tab-focusable in print preview.

chrome/browser/resources/pdf/pdf_scripting_api.js

 // 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.
 
 /**
  * Turn a dictionary received from postMessage into a key event.
  * @param {Object} dict A dictionary representing the key event.
  * @return {Event} A key event.
  */
 function DeserializeKeyEvent(dict) {
@@ skipping 245 matching lines @@
  * interface which provides access to various features of the viewer for use
  * by print preview and accessibility.
  * @param {string} src the source URL of the PDF to load initially.
  * @return {HTMLIFrameElement} the iframe element containing the PDF viewer.
  */
 function PDFCreateOutOfProcessPlugin(src) {
   var iframe = window.document.createElement('iframe');
   iframe.setAttribute(
       'src',
       'chrome-extension://mhjfbmdgcfjbbpaeojofohoefgiehjai/index.html?' + src);
+  // Prevent the frame from being tab-focusable.
+  iframe.setAttribute('tabindex', '-1');
   var client = new PDFScriptingAPI(window);
   iframe.onload = function() {
     client.setPlugin(iframe.contentWindow);
   };
   // Add the functions to the iframe so that they can be called directly.
   iframe.setViewportChangedCallback =
       client.setViewportChangedCallback.bind(client);
   iframe.setLoadCallback = client.setLoadCallback.bind(client);
   iframe.setKeyEventCallback = client.setKeyEventCallback.bind(client);
   iframe.resetPrintPreviewMode = client.resetPrintPreviewMode.bind(client);
   iframe.loadPreviewPage = client.loadPreviewPage.bind(client);
   iframe.sendKeyEvent = client.sendKeyEvent.bind(client);
   return iframe;
 }