Add witman to metrics OWNERS

Change-Id: Ia3a268d031a9696c159d4c4605c2a44bd5e37a10
Reviewed-on: https://chromium-review.googlesource.com/894105Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Commit-Queue: Mike Wittman <wittman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#533038}

components/metrics/OWNERS

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