Move some dirs from chrome/browser/chromeos/login/

session/ and signin/ are moved to chrome/browser/ash/login/.

This CL is part of the Chrome OS source code directory migration:
https://docs.google.com/document/d/1g-98HpzA8XcoGBWUv1gQNr4rbnD5yfvbtYZyPDDbkaE.

Bug: 1164001
Change-Id: I6972aaf87e78f4cffdd02eef12f03aa6ecfdad8c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2716446


Commit-Queue: Henrique Ferreiro <hferreiro@igalia.com>
Owners-Override: Henrique Ferreiro <hferreiro@igalia.com>
Reviewed-by: Hidehiko Abe <hidehiko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#858964}