GitLab

- Adds missing rules of BUILD.gn
- removes unnecessary const which causes warning for some compiler
  flag set on GN.

BUG=None
R=mpearson@chromium.org, cmasone@chromium.org
TEST=manually

Review URL: https://codereview.chromium.org/644603002

Cr-Commit-Position: refs/heads/master@{#299012}