The main purpose is to avoid running slow and useless operation in
chromium/src checkouts. Ideally, this should be done in every DEPS-ed
repo, too, but that requires substantially more changes in gclient.
Thus, this CL provides biggest bang for the buck.

Example led build: https://ci.chromium.org/swarming/task/3c62efb31ed3a910?server=chromium-swarm.appspot.com

R=hinoka@chromium.org

Bug: 822437
Change-Id: I347d85e5cb5158b77969a755b67ca2289fb39d40
Reviewed-on: https://chromium-review.googlesource.com/973900
Commit-Queue: Andrii Shyshkalov <tandrii@chromium.org>
Reviewed-by: Ryan Tseng <hinoka@chromium.org>