Fix various issues in the roll_gn script.

Fix various issues in the roll_gn script.

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=

Committed: https://crrev.com/513d94882c9720688a3e3f67da2b0a529e27b7b9
Cr-Commit-Position: refs/heads/master@{#351141}

[brettw, 2015-09-28 20:06:26]

lgtm

[Dirk Pranke, 2015-09-28 20:14:41]

The CQ bit was checked by dpranke@chromium.org

[commit-bot: I haz the power, 2015-09-28 20:16:01]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1372203003/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1372203003/20001

[Bons, 2015-09-28 20:23:38]

lgtm

[commit-bot: I haz the power, 2015-09-28 20:58:00]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-09-28 20:59:38]

Patchset 2 (id:??) landed as
https://crrev.com/513d94882c9720688a3e3f67da2b0a529e27b7b9
Cr-Commit-Position: refs/heads/master@{#351141}