Always present account chooser when signing in users via GAE Users API.
Always present account chooser when signing in users via GAE Users API.

This is same hack as used by Monorail:
https://chromium.googlesource.com/infra/infra/+/15b9f61d90423bcb2d240736e4a9401efddba42f
https://chromium.googlesource.com/infra/infra/+/0c68c1aec12aa7b0442e532eb86d1a5ca2d10728

R=hinoka@chromium.org, iannucci@chromium.org
BUG=

Review-Url: https://codereview.chromium.org/2851263003

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go