infra: Move and check gn_isolate_map. In preparation for more source-side configuration. Also adds a presubmit check and some empty files to enable the presubmit to function. Bug: angleproject:5114 Change-Id: Id9a0cbe1e78949720be39c65efab73c1345d4f39 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2819942 Reviewed-by: Yuly Novikov <ynovikov@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>