dbus: Stop accessing ObjectProxy::name_owner_changed_callback_ on the D-Bus thread

Accessing the callback on the D-Bus thread while changing its value may result in a race condition.
Change the type of SetNameOwnerChangedCallback's argument from Signal to strings to stop worrying about on which thread the Signal gets released.

BUG=298747
TEST=dbus_unittests
R=satorux@chromium.org

Review URL: https://codereview.chromium.org/24673006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@225675 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 225675