Add vmpstr and ericrk to content/renderer/gpu/OWNERS

R=kbr@chromium.org

Change-Id: I52a8430a9cd50f85da4cae49a2e784c51a63f601
Reviewed-on: https://chromium-review.googlesource.com/809534Reviewed-by: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Eric Karl <ericrk@chromium.org>
Commit-Queue: vmpstr <vmpstr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#521780}

content/renderer/gpu/OWNERS

+enne@chromium.org
+ericrk@chromium.org
 kbr@chromium.org
 piman@chromium.org
-enne@chromium.org
+vmpstr@chromium.org
 
 # COMPONENT: Internals>Compositing