dbus: Rewrite ObjectProxy::ConnectToSignal with PostTaskAndReplyWithResult

BUG=None
TEST=dbus_unittests
R=satorux@chromium.org

Review URL: https://codereview.chromium.org/24557002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@225362 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 225362