[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
TBR=noms@chromium.org
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.

Review URL: https://codereview.chromium.org/489953002

Cr-Commit-Position: refs/heads/master@{#291201}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@291201 0039d316-1c4b-4281-b951-d872f2087c98
(cherry picked from commit cc3419c1c4d0cd4fe3cfcb7bfd6c79b31e4cab0d)

Committed to pending queue: https://chromium.googlesource.com/chromium/src/+/6c1d45d

[Roger Tawa OOO till Jul 10th, 2014-08-25 17:37:31]

Committed patchset #1 to pending queue manually as 6c1d45d (presubmit
successful).