Add ericrk@ to GPU owners

Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;luci.chromium.try:linux_optional_gpu_tests_rel;luci.chromium.try:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel
Change-Id: I603c9303c30f6054c175b6ef0c21e2bbd7c0c893
Reviewed-on: https://chromium-review.googlesource.com/971162Reviewed-by: Antoine Labour <piman@chromium.org>
Commit-Queue: Eric Karl <ericrk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#547976}

gpu/OWNERS

+ericrk@chromium.org
 kbr@chromium.org
 piman@chromium.org
 vmiura@chromium.org