UI fixes to the desktop User Manager screen.

Screenshot: https://docs.google.com/file/d/0B1B1Up4p2NRMdUVvYmhWSmNZNHM/edit?usp=sharing

BUG=257033
TEST=On Windows, enable the --new-profile-management flag and chose "View all people" from the avatar menu. For Mac/Linux, pass the flag at the command line, and navigate to chrome://user-manager in the browser. The flag isn't supported on Linux and Mac, so use at your own risk :)

Review URL: https://codereview.chromium.org/25737002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@226855 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 226855