Nominating myself as a themes OWNER.

I've done enough work on them over time that I think I'm capable of reviewing
changes here.

Bug: none
Change-Id: Ia3ed68a601b2a7c55ad4cd5918c18ed5f83700a0
Reviewed-on: https://chromium-review.googlesource.com/1147663Reviewed-by: Evan Stade <estade@chromium.org>
Reviewed-by: Peter Kotwicz <pkotwicz@chromium.org>
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#577391}

chrome/browser/themes/OWNERS

 estade@chromium.org
+pkasting@chromium.org
 pkotwicz@chromium.org
 
 # COMPONENT: UI>Browser>Themes