[Remoting] Make OAuthTokenGetter::TokenCallback OnceCallback

This CL makes OAuthTokenGetter::TokenCallback OnceCallback so that it's
cleaner to use. It also marks
OAuthTokenGetter::CredentialsUpdatedCallback repeating callback, which
doesn't change its behavior but makes it cleaner.

Bug: 824488
Change-Id: I7b5a64c66792ac32b01d788f1e4e2895d97a393a
Reviewed-on: https://chromium-review.googlesource.com/c/1477924
Commit-Queue: Yuwei Huang <yuweih@chromium.org>
Reviewed-by: Joe Downing <joedow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#633930}
17 files changed