Add caitlinfischer as an owner of fieldtrial_testing_config.json.

Change-Id: Ic661aa6d85ec6a77044046ef97111b8a2fca93ab
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2464209Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Commit-Queue: Alexei Svitkine <asvitkine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#816598}

testing/variations/OWNERS

 file://base/metrics/OWNERS
 
+per-file fieldtrial_testing_config.json=caitlinfischer@google.com
+
 # Use the following owners only if:
 #  - You work in the same area as them.
 #  - They are already aware of the field trial being modified.