[platform] Enforce OWNERS for low-level platform implementations

Bug: 
No-try: true
Change-Id: I5c4f050d338cb75e93e17aa645922673c74f16e7
Reviewed-on: https://chromium-review.googlesource.com/721664Reviewed-by: Hannes Payer <hpayer@chromium.org>
Reviewed-by: Daniel Clifford <danno@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48750}

src/base/platform/OWNERS

+set noparent
+
+hpayer@chromium.org
+mlippautz@chromium.org
+
+# COMPONENT: Blink>JavaScript