Add tschumann@ to components/ntp_snippets/OWNERS

Change-Id: I54f63844b7c664553434008aafcf98e2d115d669
Reviewed-on: https://chromium-review.googlesource.com/913129
Commit-Queue: Marc Treib <treib@chromium.org>
Reviewed-by: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#536039}

components/ntp_snippets/OWNERS

 bauerb@chromium.org
 jkrcal@chromium.org
 treib@chromium.org
+tschumann@chromium.org
 
 # Unsure who to ask? Choose from the above.
 markusheintz@chromium.org