WebUI: fix cr-action-menu keyboard off-by-one issue

ui/webui/resources/cr_elements/cr_action_menu/cr_action_menu.js

 // Copyright 2016 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.
 
 Polymer({
   is: 'cr-action-menu',
   extends: 'dialog',
 
   /**
    * List of all options in this action menu.
@@ skipping 106 matching lines @@
    */
   getNextOption_: function(step) {
     // Using a counter to ensure no infinite loop occurs if all elements are
     // hidden/disabled.
     var counter = 0;
     var nextOption = null;
     var numOptions = this.options_.length;
     var focusedIndex =
         Array.prototype.indexOf.call(this.options_, this.root.activeElement);
 
+    // Avoid off-by-one when nothing is focused and up is pressed.

[dpapad, 2017/04/05 21:49:13]

Nit (optional): Perhaps just rephrase to
// Handle case where nothing is focused and up is pressed.

[scottchen, 2017/04/06 17:54:02]

Done.

+    if (focusedIndex === -1 && step === -1)
+      focusedIndex = 0;
+
     do {
       focusedIndex = (numOptions + focusedIndex + step) % numOptions;
       nextOption = this.options_[focusedIndex];
       if (nextOption.disabled || nextOption.hidden)
         nextOption = null;
       counter++;
     } while (!nextOption && counter < numOptions);
 
     return nextOption;
   },
@@ skipping 39 matching lines @@
     // Attempt to show the menu starting from the top of the rectangle and
     // extending downwards. If that does not fit within the window, fallback to
     // starting from the bottom and extending upwards.
     var top = rect.top + this.offsetHeight <= window.innerHeight ? rect.top :
                                                                    rect.bottom -
             this.offsetHeight - Math.max(rect.bottom - window.innerHeight, 0);
 
     this.style.top = top + 'px';
   },
 });