List Weilun as a fieldtrial_testing_config.json OWNER

Weilun is sufficiently trained up for reviewing testing config changes. Will
eventually remove this entry in favor of listing as a general metrics OWNER, but
this is a stepping stone in the meantime.

R=sweilun

Bug: None
Change-Id: If40902f38b8473b6a46eba6e08d2ed8088156838
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2522398
Auto-Submit: Ilya Sherman <isherman@chromium.org>
Commit-Queue: Weilun Shi <sweilun@chromium.org>
Reviewed-by: Weilun Shi <sweilun@chromium.org>
Cr-Commit-Position: refs/heads/master@{#824680}

testing/variations/OWNERS

 file://base/metrics/OWNERS
 
+# Temporarily listed here while training up as broader metrics owners.
 per-file fieldtrial_testing_config.json=caitlinfischer@google.com
+per-file fieldtrial_testing_config.json=sweilun@chromium.org
 
 # Use the following owners only if:
 #  - You work in the same area as them.