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

This is interfering slightly with tab ordering. The plugin never used to be tab
focusable.

BUG=455552

Review URL: https://codereview.chromium.org/889603005

Cr-Commit-Position: refs/heads/master@{#314962}

chrome/browser/resources/pdf/pdf_scripting_api.js

@@ -263,6 +263,8 @@ function PDFCreateOutOfProcessPlugin(src) {
   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);