Add Dominik Röttsches to platform OWNERS

Add dominik.rottsches@intel.com to Source/platform/OWNERS, Dominik is a
key contributor to the font code and de-facto owner of much of the Mac
specific font and text code.

R=abarth@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/348363004

git-svn-id: svn://svn.chromium.org/blink/trunk@176949 bbb929c8-8fbe-4397-9dbb-9b2b20218538