Add OWNERS for blink/renderer/core/origin_trials

Change-Id: I21b76fb4be0fdde050d670d521f3192a392921b4
Reviewed-on: https://chromium-review.googlesource.com/c/1443313
Auto-Submit: Ian Clelland <iclelland@chromium.org>
Reviewed-by: Steve Kobes <skobes@chromium.org>
Commit-Queue: Steve Kobes <skobes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#627024}

third_party/blink/common/origin_trials/OWNERS

 # This file also covers ownership of the following directories:
 #   //chrome/common/origin_trials/
 #   //third_party/blink/public/common/origin_trials/
+#   //third_party/blink/renderer/core/origin_trials/
 #   //tools/origin_trials/
 
 chasej@chromium.org

third_party/blink/renderer/core/origin_trials/OWNERS

+file://third_party/blink/common/origin_trials/OWNERS