Assign event listener to each link in the frame doc instead on the whole doc (which doesn't seem to work in talk-throughs)

Test Plan:
- Load /computing/computer-programming/html-css/html-tags-continued/p/html-links
- Play to 2/3 of the way through, click on the link, see the alert and it open up.
- Did same in Chrome and Firefox.

Auditors: john