GitLab

To decrease UKM volume, this maintains a list of the 20 most recently
recorded (source, target) UKM source ID pairs, such that duplicates can
be skipped.

Bug: 1112491
Change-Id: I4b6bfe26f55102078305bc7a5c3bc1a774ca62d2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2622277Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Aaron Colwell <acolwell@chromium.org>
Commit-Queue: Lily Chen <chlily@chromium.org>
Cr-Commit-Position: refs/heads/master@{#842718}