Add benmason@ to android_webview/tools OWNERS

Specifically for the file apk_merger.py

Bug: None
Change-Id: I0e889d95d7749dbce3d812fc971e2b690ab20789
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1896506
Commit-Queue: Nate Fischer <ntfschr@chromium.org>
Reviewed-by: Nate Fischer <ntfschr@chromium.org>
Auto-Submit: Peter Wen <wnwen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#712230}

android_webview/tools/OWNERS

 ntfschr@chromium.org
 
+per-file apk_merger.py=benmason@chromium.org
+
 # COMPONENT: Mobile>WebView