Add mpdenton to sandbox/linux/OWNERS.

Change-Id: If61501127fe7041b3a1194ef1aad8c56e6920de4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2095407Reviewed-by: Robert Sesek <rsesek@chromium.org>
Reviewed-by: Matthew Denton <mpdenton@chromium.org>
Commit-Queue: Chris Palmer <palmer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#749266}

sandbox/linux/OWNERS

 jorgelo@chromium.org
+mpdenton@chromium.org
 palmer@chromium.org
 rsesek@chromium.org