[libFuzzer] Make metzman@chromium.org an owner of third_party/libFuzzer

Bug: 
Change-Id: Ica0408b417f3ac56c1e90989a28d998735e7fc04
Reviewed-on: https://chromium-review.googlesource.com/830806Reviewed-by: Abhishek Arya <inferno@chromium.org>
Commit-Queue: Abhishek Arya <inferno@chromium.org>
Cr-Commit-Position: refs/heads/master@{#524461}

third_party/libFuzzer/OWNERS

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