libaom/libvpx: add jzern@ as OWNER

R=tomfinegan@chromium.org

Change-Id: I926b4d76701861c4351baf6e4982ce0beb962e49
Reviewed-on: https://chromium-review.googlesource.com/1153584Reviewed-by: James Zern <jzern@chromium.org>
Commit-Queue: Johann Koenig <johannkoenig@google.com>
Cr-Commit-Position: refs/heads/master@{#578763}

third_party/libaom/OWNERS

 tomfinegan@chromium.org
 johannkoenig@google.com
 jzern@chromium.org
+jzern@google.com

third_party/libvpx/OWNERS

 tomfinegan@chromium.org
 johannkoenig@google.com
+jzern@chromium.org
+jzern@google.com