Fix various stray bugs in the GN auto-roller, and repair the DEPS file.

Fix various stray bugs in the GN auto-roller, and repair the DEPS file.

The GN auto-roller script had two bugs in it, one of which was
damaging the DEPS file: when we updated the buildtools revision,
we would drop a newline and end up concatenating the following line.
A second roll would then drop that concatenated line, shrinking the
file, and so one. This was bad.

Secondly, the auto-roller would leave buildtools/ checked out on a
branch that had been merged into origin/master; this might cause
merge conflicts and confusion down the road.

R=thakis@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1289233003

Cr-Commit-Position: refs/heads/master@{#343804}