Fix SHIFT+TAB order on GAIA login screens

Fix SHIFT+TAB order on GAIA login screens

BUG=488664
TEST=SHIFT+TAB repeatedly through login screen.

When focus is on the system tray, SHIFT+TAB should move focus onto the
[...] button, or whatever is the previous button on the shelf.
Previously, TAB and SHIFT+TAB would both move focus onto "Enter your
email" (or somewhere else in the GAIA elements).

Review URL: https://codereview.chromium.org/1173083003

Cr-Commit-Position: refs/heads/master@{#333803}
(cherry picked from commit eb2fad4839df15e0fee76f6a4f7beebf0244e167)
NOTRY=true
NOPRESUBMIT=true

[Greg Levin, 2015-06-11 17:58:32]

glevin@chromium.org changed reviewers:
+ xiyuan@chromium.org

[Greg Levin, 2015-06-11 17:59:36]

Please have a quick look for 44 merge

[xiyuan, 2015-06-11 18:21:25]

LGTM

Does CQ handle landing on branch?

[Greg Levin, 2015-06-11 19:27:50]

The CQ bit was checked by glevin@chromium.org

[commit-bot: I haz the power, 2015-06-11 19:29:42]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1178203003/1

[commit-bot: I haz the power, 2015-06-11 19:33:01]

Committed patchset #1 (id:1)