Improve the identity::SetRefreshForXXXAccount() API

Replace the pointer to a token value by a const reference with empty
string as default value. Fixed also the documentation.

This is a follow-up of crrev.com/c/1349983

Change-Id: Ie07a0c346177a48589ebac5797b0e84e86cf347b
Reviewed-on: https://chromium-review.googlesource.com/c/1371877
Commit-Queue: Sergio Villar <svillar@igalia.com>
Reviewed-by: Colin Blundell <blundell@chromium.org>
Reviewed-by: Alexander Alekseev <alemate@chromium.org>
Cr-Commit-Position: refs/heads/master@{#616257}
4 files changed