Add Gitmodules to angle. Commits in the gitlinks should be kept in sync with commits in DEPS. For repos that are auto-rolled by skia, this is taken care of. If you have your own autoroll script and it uses `gclient setdep` to update deps commits, this is also taken care of. For manual rolls: 1) in DEPS update the commit of some <path> to <new_commit> 2) update the gitlink: `git update-index --add --cacheinfo 160000,<new_commit>,<path>` Bug: chromium:1421776 Change-Id: I70d02969b198ea6ae94d042c4ffab31b70eb88fb Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/4752314 Commit-Queue: Jonah Ryan-Davis <jonahr@google.com> Reviewed-by: Jonah Ryan-Davis <jonahr@google.com> Commit-Queue: Joanna Wang <jojwang@chromium.org>