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

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

Index: ui/webui/resources/cr_elements/cr_action_menu/cr_action_menu.js
diff --git a/ui/webui/resources/cr_elements/cr_action_menu/cr_action_menu.js b/ui/webui/resources/cr_elements/cr_action_menu/cr_action_menu.js
index a8e065db682e77c39cd3f2a1cb2e694e684c8ea7..2a75c307095dd2ee3264e614c2f0075af304a878 100644
--- a/ui/webui/resources/cr_elements/cr_action_menu/cr_action_menu.js
+++ b/ui/webui/resources/cr_elements/cr_action_menu/cr_action_menu.js
@@ -124,6 +124,10 @@ Polymer({
     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];