Add a test for mime handler view iframes and make them work on Windows.

chrome/test/data/extensions/api_test/mime_handler_view/test_iframe.js

+// Copyright 2015 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.
+
+var iframe = document.getElementById('iframe');
+
+function printIframe() {
+  iframe.contentWindow.print();
+}
+
+if (iframe.contentDocument && iframe.contentDocument.body &&
+    iframe.contentDocument.body.firstChild) {
+  printIframe();
+} else {
+  iframe.onload = printIframe;
+}