Add additional owners to media/gpu/vaapi/OWNERS

To reflect reviewer specialization/code familiarity per OWNERS
file/reviewer choice guidelines.

BUG=None
TEST=None

Cq-Include-Trybots: master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel
Change-Id: I0a3bb4cee242c9a3e59bb451ea697889d1679856
Reviewed-on: https://chromium-review.googlesource.com/821150Reviewed-by: Miguel Casas <mcasas@chromium.org>
Commit-Queue: Pawel Osciak <posciak@chromium.org>
Cr-Commit-Position: refs/heads/master@{#523282}

media/gpu/vaapi/OWNERS

+kcwu@chromium.org
 mcasas@chromium.org
+posciak@chromium.org