Add a chromium author that could not add himself

BUG=1116202

Change-Id: I10738961da81ad153b770210e3d969210b0bcd0f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2354557
Commit-Queue: Donn Denman <donnd@chromium.org>
Commit-Queue: Theresa  <twellington@chromium.org>
Auto-Submit: Donn Denman <donnd@chromium.org>
Reviewed-by: Theresa  <twellington@chromium.org>
Cr-Commit-Position: refs/heads/master@{#797916}

AUTHORS

@@ -721,6 +721,7 @@ Nagarajan Narayanan <nagarajan.n@samsung.com>
 Nagarjuna Atluri <nagarjuna.a@samsung.com>
 Naiem Shaik <naiem.shaik@gmail.com>
 Naoki Takano <takano.naoki@gmail.com>
+Nathan Mitchell <nathaniel.v.mitchell@gmail.com>
 Naveen Bobbili <naveenbobbili@motorola.com>
 Naveen Bobbili <qghc36@motorola.com>
 Naveen Kumar Devaraj <devarajn@amazon.com>