Add per-file OWNERS for signin-related files in //components.

The list of OWNERS added is the same as that of //components/signin/OWNERS.

R=joi@chromium.org

Review URL: https://codereview.chromium.org/142973006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@246014 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 246014