First try at a user management screen for the desktop. 
This should look and have similar functionality as the CrOS account picker 
screen. The implementation cannot be fully merged, however, as some actions do 
not make sense in a desktop sense (for example shut down), and the server 
code is slightly different. 
More details can be found in this design document:
https://docs.google.com/a/google.com/document/d/1fg47-s19akfqaKNqjggS6Igj_pkXExhgTgdVPl2BZ7Y/edit?usp=sharing
The desktop user chooser can be accessible by enabling the 
--new-profile-management flag and going to chrome://user-chooser. It is not 
available on CrOs/iOS/Android. In a future CL, it will be linked to the avatar 
menu, and will come up when users sign out. 
This code is *not* meant to replace the CrOS account picker, and should not 
be used in CrOS. 
Implementation quirks: 
1) Guest-mode is currently not ready for the desktop, so "Browse as Guest" 
brings up an Incognito browser. 
2) If a profile is signed-out/locked, the corresponding pod shows a 
password field, but this password is not actually validated, and the profile 
browser is launched. This can't be fixed at the moment, but will be 
fixed in a future CL. 
3) There are no high-resolution avatar resources. Until this problem is solved, 
we are displaying a standard "chrome user" avatar.
This will be fixed in a future CL with either higher resolution resources
or the CrOS profile picture chooser dialog.
BUG=
257033
TEST=Enable --new-profile-management. Go to chrome://user-chooser. 
This should display a row of user pods, for each existing profile.
Clicking on the pods should either give you a password field, or a
"Sign in" button, both of which should launch a new browser window 
for the selected profile (no password authentication is done at the moment). 
Screenshot: 
https://docs.google.com/a/chromium.org/file/d/0B1B1Up4p2NRMMHRMaHlITGxRdUk/edit?usp=sharing
RTL screenshot:
https://docs.google.com/a/chromium.org/file/d/0B1B1Up4p2NRMMVFrZUdnQjBISkU/edit?usp=sharing
Committed: 
https://src.chromium.org/viewvc/chrome?view=rev&revision=212688