After switching bots from GYP to GN, build artifacts are left that fails
the next builds. Since it's unfeasible to clean out all bot machines
it's better to have an automated system for this, which is what landmines is.

By adding a line to tools/get_landmines.py it is possible to clobber each bot
that syncs past that "landmine CL".

BUG=chromium:652188
TBR=ehmaldonado@chromium.org

Review URL: https://codereview.chromium.org/2427633003 .