[AFL] Make metzman@chromium.org an OWNER of third_party/afl

Bug: 
Change-Id: I8f9d5efa70f298fe523fc3caf1d4b1baacd8ebae
Reviewed-on: https://chromium-review.googlesource.com/828714Reviewed-by: Jonathan Metzman <metzman@chromium.org>
Reviewed-by: Max Moroz <mmoroz@chromium.org>
Commit-Queue: Max Moroz <mmoroz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#524405}

third_party/afl/OWNERS

 inferno@chromium.org
 kcc@chromium.org
+metzman@chromium.org
 mmoroz@chromium.org
 ochang@chromium.org