Add OWNERS for base/test/*fontconfig*

Change-Id: Iec9648c228c63be6d46fe3a6db9c8bc84780dfaf
Reviewed-on: https://chromium-review.googlesource.com/973697Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#544763}

base/test/OWNERS

@@ -8,3 +8,6 @@ per-file *user_action_tester*=file://base/metrics/OWNERS
 # For Android-specific changes:
 per-file *android*=file://base/test/android/OWNERS
 per-file BUILD.gn=file://base/test/android/OWNERS
+
+# Linux fontconfig changes
+per-file *fontconfig*=file://base/nix/OWNERS