Add vmpstr as a core owner

Also, clean up an obsolete comment.

Change-Id: I489387a7ed13edc777fd11c30deadac962affe6e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2364217Reviewed-by: vmpstr <vmpstr@chromium.org>
Commit-Queue: Chris Harrelson <chrishtr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#799753}

third_party/blink/renderer/core/OWNERS

@@ -69,7 +69,7 @@ thakis@chromium.org
 timloh@chromium.org
 tkent@chromium.org
 tonyg@chromium.org
-# vollick reviews compositor integration, accelerated transitions, animations and scrolling, scrolling coordinator, CSS transforms
+vmpstr@chromium.org
 vollick@chromium.org
 wangxianzhu@chromium.org
 yhirano@chromium.org