Unify the 'log a user into a webview with oauth' pattern -- video oauth now uses same process as exercise oauth.

Test Plan:
Start watching a video while logged in
Pause video
Verify that points are properly recorded
Log out
Verify that points are no longer recorded while watching
Log back in
Verify that points are recorded again.

Reviewers: alpert

Reviewed By: alpert

Subscribers: laura

Differential Revision: http://phabricator.khanacademy.org/D9552