ChromeOS OOBE: fix JS crash in oobe-dialog focus behavior.

BUG=707939
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2818633003
Cr-Commit-Position: refs/heads/master@{#464609}

chrome/browser/resources/chromeos/login/oobe_dialog.js

@@ -39,7 +39,7 @@ Polymer({
     */
   show: function() {
     var focusedElements = this.getElementsByClassName('focus-on-show');
-    if (focusedElements)
+    if (focusedElements.length > 0)
       focusedElements[0].focus();
 
     this.fire('show-dialog');