Portals: Add OWNERS file for portals directories.

Change-Id: I018d44da16943973206e04084f85aa3793bec79f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1791626Reviewed-by: Nasko Oskov <nasko@chromium.org>
Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Commit-Queue: Lucas Gadani <lfg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#695697}

content/browser/portal/OWNERS

+file://third_party/blink/renderer/core/html/portal/OWNERS
+
+# COMPONENT: Blink>HTML>Portal

third_party/blink/renderer/core/html/portal/OWNERS

+adithyas@chromium.org
+jbroman@chromium.org
+lfg@chromium.org
+mcnee@chromium.org
+
+# COMPONENT: Blink>HTML>Portal