Application Version (from about://version): Google Chrome 50.0.2661.91 (official 0)
Device: Chromebook

Steps to reproduce: 
Have Data Saver enabled on a network that allows the use of the SSL proxy (i.e. the secure proxy check succeeded), then switch to a network with a captive portal that sends redirects (such as Dallas Ft Worth airport WiFi), with Data Saver still enabled.

Observed behavior: 
The secure proxy check returns a 302 redirect to the CP login page, but Chrome ignores this and continues to try using the SSL data reduction proxy even though it won't work.

Expected behavior: 
The secure proxy check returns a 302 redirect to the CP login page, which should cause Chrome to stop attempting to use the SSL data reduction proxy, allowing the user to fall back to the non-SSL proxy and get through the CP login page.