[Mac] New avatar bubble should be a fixed width

This involves eliding ALL the things: the active profile
name, the names of other profiles in the fast profile
switcher, and the text of the big blue "add account for x"
button.

BUG=334761

Review URL: https://codereview.chromium.org/133213017

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@250456 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 250456