GitLab

When |rlz_string| is non-empty, we can build the header and simply
return it ourselves.

This will help us to fix the Darin's TODO in HttpUtil to remove
HasHeader() method.

BUG=None
TEST=components_unittests
R=mmenke@chromium.org,rogerta@chromium.org

Review-Url: https://codereview.chromium.org/2621843002
Cr-Commit-Position: refs/heads/master@{#442616}