Add wittman to third_party/metrics OWNERS

Change-Id: If1376c69b262c9112f8c3653feffebe961bd92b6
Reviewed-on: https://chromium-review.googlesource.com/894487Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Commit-Queue: Alexei Svitkine <asvitkine@chromium.org>
Commit-Queue: Mike Wittman <wittman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#533044}

third_party/metrics_proto/OWNERS

 file://base/metrics/OWNERS
 
+per-file sampled_profile.proto=wittman@chromium.org
+per-file call_stack_profile.proto=wittman@chromium.org
+per-file execution_context.proto=wittman@chromium.org
+
 # COMPONENT: Internals>Metrics