The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/66e5881e60de5d905202275ce1f7edb804e6c786

commit 66e5881e60de5d905202275ce1f7edb804e6c786
Author: Mike Dougherty <michaeldo@chromium.org>
Date: Fri Nov 03 06:54:28 2017

Open Captive Portal Login page in a new tab instead of modal UI.

Using a new tab is consistent with other platforms, and solves security
concerns with the modal by eliminating a new UI surface. 

Bug:  779587 ,  758252 
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: I05d1db266fb93493c010661d567b3ae15a9929e9
Reviewed-on: https://chromium-review.googlesource.com/740365
Commit-Queue: Mike Dougherty <michaeldo@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#513714}
[modify] https://crrev.com/66e5881e60de5d905202275ce1f7edb804e6c786/ios/chrome/browser/ssl/BUILD.gn
[modify] https://crrev.com/66e5881e60de5d905202275ce1f7edb804e6c786/ios/chrome/browser/ui/BUILD.gn
[modify] https://crrev.com/66e5881e60de5d905202275ce1f7edb804e6c786/ios/chrome/browser/ui/browser_view_controller.mm
[delete] https://crrev.com/b7c153bd16db1cffae948a57315df053531a530a/ios/chrome/browser/ui/captive_portal/BUILD.gn
[delete] https://crrev.com/b7c153bd16db1cffae948a57315df053531a530a/ios/chrome/browser/ui/captive_portal/captive_portal_login_coordinator.h
[delete] https://crrev.com/b7c153bd16db1cffae948a57315df053531a530a/ios/chrome/browser/ui/captive_portal/captive_portal_login_coordinator.mm
[delete] https://crrev.com/b7c153bd16db1cffae948a57315df053531a530a/ios/chrome/browser/ui/captive_portal/captive_portal_login_coordinator_unittest.mm
[delete] https://crrev.com/b7c153bd16db1cffae948a57315df053531a530a/ios/chrome/browser/ui/captive_portal/captive_portal_login_view_controller.h
[delete] https://crrev.com/b7c153bd16db1cffae948a57315df053531a530a/ios/chrome/browser/ui/captive_portal/captive_portal_login_view_controller.mm
[delete] https://crrev.com/b7c153bd16db1cffae948a57315df053531a530a/ios/chrome/browser/ui/captive_portal/captive_portal_login_view_controller_delegate.h
[modify] https://crrev.com/66e5881e60de5d905202275ce1f7edb804e6c786/ios/chrome/test/BUILD.gn

Won't this lead to a proliferation of needless additional tabs for users? 

Mardini, what do you think?

It shouldn't. A new tab will only open if the user taps on "Connect" and the newly created tab can be used just like any other tab. I suppose in the worst case where the user doesn't know what tabs are or how to close them, the number of tabs will grow, but that should probably be addressed separately through user education.

On other platforms, the CaptivePortalTabHelper tracks if that tab is a captive portal login page. If we wanted to, we could do something similar and then close the login tab once the user connects to the internet. However, we would need to prompt the user in some way and ask if they wanted to close the tab because even after the network connects, we don't know when the user is done with the login page. (Some captive portals show additional information after the login is complete.)

I will update the design doc with the new flow and with potential options for cleaning up the "login" tab.

What michaeldo@ said. 
Pink: I added you to the thread where we entertained the various options.