GitLab

Move the LayoutInstability test case and CLS-specific stuff into a
separate file, add comments to document MetricIntegrationTest APIs,
and create UKM/UMA tester objects automatically.

Bug: 1045064
Change-Id: I905975afc97fa7ebd3b3d5382b7c77429628c07a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2064780Reviewed-by: Annie Sullivan <sullivan@chromium.org>
Commit-Queue: Steve Kobes <skobes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#743099}