GitLab

Provide a global constant for a null unguessable token, for functions
that need to return a const base::UnguessableToken&.

Change-Id: Ib07dde7862dcab19baac409eaecda3482cfc5f4d
Reviewed-on: https://chromium-review.googlesource.com/1162986Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#580761}