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Currently all available perception results are added to the perception
sample as entities. When there are no entities, we dont know if there
were no entities detected, or no detections were run at all.

BUG=872764

Change-Id: I18d4b0f1a0e285f030e5d6f64a4c62c0dc23ae07
Reviewed-on: https://chromium-review.googlesource.com/1169311Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Reviewed-by: Toni Baržić <tbarzic@chromium.org>
Commit-Queue: Wei Guan <weigua@chromium.org>
Cr-Commit-Position: refs/heads/master@{#582994}