GitLab

This patch address a bunch of issues people have found in the roll_gn script:

- Fixes yet another bug where we weren't waiting for the try jobs
  to finish.
- Fixes a bug where we were adding an extra '\n' onto the DEPS file
  in the final roll CL.
- Closes the 'build_gn' CL once the build has completed.
- Adds better logging at the end of the 'wait' and 'roll_buildtools' steps.
- Removes mac_chromium_gn_rel from CQ_EXTRA_TRYBOTS, since
  it is part of the default set now.

R=brettw@chromium.org, andybons@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1372203003

Cr-Commit-Position: refs/heads/master@{#351141}