GitLab

The public PGO builders do not do chrome-branded builds, so
can't reuse the existing 'official' mixin and need to split out
is_official_build from is_chrome_branded.

TBR=sebmarchand@chromium.org
NOTRY=true
BUG=595947

Review-Url: https://codereview.chromium.org/2180513002
Cr-Commit-Position: refs/heads/master@{#407230}