Add dedicated OWNERS for WebAPK metrics

This CL makes chrome/browser/webapps OWNERS also OWNERS of WebAPK
metrics in chrome/browser/metrics

BUG=None

Change-Id: I874c8560e0bfd393ad85b3ff85030d668bb2b993
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1733115Reviewed-by: Yaron Friedman <yfriedman@chromium.org>
Commit-Queue: Yaron Friedman <yfriedman@chromium.org>
Auto-Submit: Peter Kotwicz <pkotwicz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#683653}

chrome/android/java/src/org/chromium/chrome/browser/metrics/OWNERS

+per-file WebApk*=file://chrome/android/java/src/org/chromium/chrome/browser/webapps/OWNERS