Add OWNERS file for //base/containers

Bug: none
Change-Id: I30f3a590c9021dafbf5276621a8e27a8477f703f
Reviewed-on: https://chromium-review.googlesource.com/585507Reviewed-by: danakj <danakj@chromium.org>
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#490814}

base/containers/OWNERS

+brettw@chromium.org
+danakj@chromium.org
+dcheng@chromium.org
+vmpstr@chromium.org