[signin] Untrusted signin dialog is moved earlier in the signin flow

This CL moves the untrusted signin dialog from the sync starter to
the inline login handler.
This will allow removing the sync starter code in a followup.

Bug: 915678
Change-Id: Id7422813e72ca31c2ec6ddc2b42deef33e363d1c
Reviewed-on: https://chromium-review.googlesource.com/c/1401065
Commit-Queue: David Roger <droger@chromium.org>
Reviewed-by: Mihai Sardarescu <msarda@chromium.org>
Cr-Commit-Position: refs/heads/master@{#622860}
7 files changed