[signin] Cleanup embedded URL API in signin_promo

This CL removes unused functions, moves the ChromeOS specific code
under OS_CHROMEOS guards, and adds a "Embedded" prefix to the
functions.

TBR=ellyjones, tangltom

Bug: 915678
Change-Id: Ica5e7ee5c40cb6fc40f3a9d09533357f21ae441a
Reviewed-on: https://chromium-review.googlesource.com/c/1411593
Reviewed-by: David Roger <droger@chromium.org>
Reviewed-by: Mihai Sardarescu <msarda@chromium.org>
Commit-Queue: David Roger <droger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#623327}
21 files changed