Move SigninNamesOnIOThread to observing SigninManager.

Unchanged reland of r251581, which was speculatively reverted to see if it was
the cause of crbug.com/345757 (which it is not). Original commit message
follows.

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 observ of
any SigninManagers that it is currently observing and removes itself as a
observer of SigninManagerFactory.

BUG=333997
TBR=rogerta

Review URL: https://codereview.chromium.org/179003003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@254457 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 254457