Fix action menu a11y tree

Currently, CrActionMenu's a11y tree looks like:

Menu
  Group (wrapper div)
    Menu item 1
    Menu item
    ...

On macOS, this causes VoiceOver to interpret the wrapper as the sole
item in the menu, and to read the wrapper's generated a11y name, which
is the concatenation of the menu items.

This change removes the wrapper from the a11y hierarchy. Removing the
tabindex is necessary since an explicitly set tabindex overrides
`role=none`.

Bug: 947222
Change-Id: Ice8124b1b19ce04773f8fe7e0fda54755fdcfd3d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1776223Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Commit-Queue: Leonard Grey <lgrey@chromium.org>
Cr-Commit-Position: refs/heads/master@{#692085}

ui/webui/resources/cr_elements/cr_action_menu/cr_action_menu.html

@@ -65,7 +65,7 @@
     </style>
     <dialog id="dialog" tabindex="0" on-close="onNativeDialogClose_" role="menu"
         aria-label$="[[ariaLabel]]">
-      <div class="item-wrapper" tabindex="-1">
+      <div class="item-wrapper" role="none">
         <slot id="contentNode"></slot>
       </div>
     </dialog>

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

@@ -358,6 +358,7 @@ Polymer({
           anchorAlignmentX: AnchorAlignment.BEFORE_END,
         },
         opt_config)));
+    this.$.dialog.focus();
   },
 
   /**