Remove usage of programmatic_auth Gaia endpoint.

Chrome desktop and Chrome OS use the Gaia programmatic_auth endpoint
for signing in users to Chrome. This endpoint is currently being
deprecated on the Google infrastructure.

The programmatic_auth endpoint was only used for the full-tab
sign-in page, which was presented in rare cases (sign-in flow starting
from apps / extensions / incognito / some particular case from settings).

This CL adds an experiment to remove the usage of the programmatic_auth
endpoint from Chrome desktop. It changes the full-tab sign-in page to
present the embedded GAIA web sign-in page.

Internal design doc:
https://docs.google.com/a/google.com/document/d/1cnoG1Jo-CZZVlFULOYrcJLoyTmID-gCgkXkT5hW2tpg/edit?usp=sharing

Bug: 775009
Change-Id: Ibfdec11972674228250b4d803e23f029a49d8cb9
Reviewed-on: https://chromium-review.googlesource.com/737994
Reviewed-by: Trent Apted <[email protected]>
Reviewed-by: Xiyuan Xia <[email protected]>
Reviewed-by: David Roger <[email protected]>
Commit-Queue: Mihai Sardarescu <[email protected]>
Cr-Commit-Position: refs/heads/master@{#512469}
18 files changed