Chrome Cleaner: collect srt field trial group name

Bug: 
Change-Id: Ib77d051ef0813cd63eafd92b578919f46982d26b
Reviewed-on: https://chromium-review.googlesource.com/700992
Reviewed-by: Chris Sharp <csharp@chromium.org>
Commit-Queue: Fabio Tirelo <ftirelo@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#506527}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: fd6e1fed2d4f14dd51d3980f263fc8d237729e01