Fix another bug in the gn_roll script.

Fix another bug in the gn_roll script.

We were not checking out the branch used to build gn before
checking on the tryjob statuses for that branch.

R=brettw@chromium.org
BUG=

Committed: https://crrev.com/3862a3e5baf3e0e5c867d82df6615414ae914855
Cr-Commit-Position: refs/heads/master@{#352145}

[brettw, 2015-10-02 21:00:47]

lgtm

[Dirk Pranke, 2015-10-02 21:09:35]

The CQ bit was checked by dpranke@chromium.org

[commit-bot: I haz the power, 2015-10-02 21:11:50]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1380373002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1380373002/1

[commit-bot: I haz the power, 2015-10-02 21:54:22]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-10-02 21:55:22]

Patchset 1 (id:??) landed as
https://crrev.com/3862a3e5baf3e0e5c867d82df6615414ae914855
Cr-Commit-Position: refs/heads/master@{#352145}