[Mac] User manager should open on the screen that has focus

[Mac] User manager should open on the screen that has focus

(vs always opening on the first screen like it's happening now)

BUG=405602
TEST=Start Chrome with --enable-new-avatar-menu (if it's not own by default). If you've got to screens, when you press "Switch users" the User Manager should show up on whatever screen you triggered it from.

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=291201

[noms (inactive), 2014-08-20 20:55:45]

Because I didn't know better at the time, the User Manager always opened on the
first screen, which is pretty annoying if you've got more than one screen. Soooo
we shouldn't do that. :)

Please take a look. Thanks!

[Robert Sesek, 2014-08-20 22:14:23]

lgtm

[noms (inactive), 2014-08-21 14:04:35]

The CQ bit was checked by noms@chromium.org

[commit-bot: I haz the power, 2014-08-21 14:05:36]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/noms@chromium.org/489953002/1

[commit-bot: I haz the power, 2014-08-21 20:04:33]

FYI, CQ is re-trying this CL (attempt #1).
The failing builders are:
  android_aosp on tryserver.chromium.linux
(http://build.chromium.org/p/tryserver.chromium.linux/builders/android_aosp/bu...)

[commit-bot: I haz the power, 2014-08-21 21:45:21]

Committed patchset #1 (1) as 291201