(bug 43909) (mingle 377) Invoke UserLoginComplete hook to watch an article after login

If a non-logged in user requested to watch an article, and then
tapped 'login' from the resulting drawer, then logged in, this
will cause the article to actually be watched immediately after
successful login, by processing the watch request when the
UserLoginComplete hook runs.

Change-Id: I5c1a1c1fb9cf0e3ee3cb754b593d50368bbe4784
Merge "(bug 43909) (mingle 377) Invoke UserLoginComplete hook to watch an article after login"