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Previously, it would only take effect when a single feature was
enabled on the study. The new logic would associate with all the
features.

Bug: 816696
Change-Id: Ide5ace721ade5e78f8743b11a96998d3b08ded46
Reviewed-on: https://chromium-review.googlesource.com/963087
Commit-Queue: Alexei Svitkine <asvitkine@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543552}