Move SigninNamesOnIOThread to observing SigninManager.

As SigninNamesOnIOThread is currently listening for signin-related
notifications from all Profiles, this change is non-trivial. This CL adds the
ability to observe SigninManagerFactory's creation of SigninManagers.
SigninNamesOnIOThread uses this hook to add itself as an observer of every
created SigninManager, removing itself as an observer of a particular
SigninManager when that SigninManager is shut down. At the time that
SigninNamesOnIOThread releases its resources, it removes itself as an observer
of any SigninManagers that it is currently observing and removes itself as an
observer of SigninManagerFactory.

BUG=333997
TBR=thakis

Review URL: https://codereview.chromium.org/132743004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251581 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 251581