online account addition fails behind captive portals

[Selene ToyKeeper]

Our online accounts system behaves in undesirable ways when trying to connect through a captive portal -- a blank white screen instead of something useful.

Steps to reproduce this issue:

1. Connect to a wifi network which uses a captive portal. Most hotels or airports and many restaurants or coffee shops work this way -- unencrypted wifi which requires authenticating or paying through a portal page before outside access will be enabled.
2. Don't launch the regular web browser or otherwise auth through the captive portal.
3. Try to run a web app such as facebook or gmail, or add an online account which hasn't been configured yet.

Expected results: Account addition UI should display the carrier's portal page or an error.

Actual results: Account addition UI gets stuck on a blank white screen.

I found this on krillin rtm image 119.