Adding OWNERs for test fonts

Bug: 1164055
Change-Id: Id74b628ce9a99e8a317d881c20a391ece1b85af9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2620620Reviewed-by: Dominik Röttsches <drott@chromium.org>
Commit-Queue: Dominik Röttsches <drott@chromium.org>
Cr-Commit-Position: refs/heads/master@{#842459}

third_party/blink/web_tests/third_party/OWNERS

+file://third_party/test_fonts/OWNERS
+