unrelatedly, if the last item is focused, Shift+Tab wraps to the first item instead of moving backward in the tab order.

I've got an in-progress CL to remove the tab cycling and switch back to paper-listbox with up/down to move through the menu: https://codereview.chromium.org/2272553002/

The CL is stalled because a) I'm focused on performance right now and b) getting the accessibility right is hard, but it would fix these problems.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/a0940bc1fbbe088a791c61b7f52e0a63930f6c4f

commit a0940bc1fbbe088a791c61b7f52e0a63930f6c4f
Author: michaelpg <michaelpg@chromium.org>
Date: Tue Sep 20 02:43:15 2016

Fix cr-shared-menu focus issues.

cr-shared-menu uses special logic to keep focus within the expanded menu. This
should not target non-focuscable items. There is also a bug where this logic
wraps to the fist element instead of moving backwards when Shift+Tab is pressed
on the last element.

BUG= 648471 
R=tsergeant@chromium.org
TEST=CrElementsSharedMenuTest.All
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2356643003
Cr-Commit-Position: refs/heads/master@{#419657}

[modify] https://crrev.com/a0940bc1fbbe088a791c61b7f52e0a63930f6c4f/chrome/test/data/webui/cr_elements/cr_shared_menu_tests.js
[modify] https://crrev.com/a0940bc1fbbe088a791c61b7f52e0a63930f6c4f/ui/webui/resources/cr_elements/cr_shared_menu/cr_shared_menu.js

 Issue 641859  has been merged into this issue.