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This CL is aimed at solving the bug described in comment 7 in crbug.com/468969. The original method only considered the system tray alone.

After the fix:

On signin screen, tabing focus order is:
three dots -> system tray -> (virtual keyboard) -> user login
shift-tabing focus order is:
user login -> (virtual keyboard) -> system tray -> three dots

On locked screen, tabing focus order is:
signout -> system tray -> notification tray -> (virtual keyboard) -> user login
reverse tabing focus order is:
user login -> (virtual keyboard) -> notification tray -> system tray -> signout

BUG=468969
TEST=device test, works good on the above description.

Review-Url: https://codereview.chromium.org/2295843006
Cr-Commit-Position: refs/heads/master@{#419234}