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This patch adds a check to ensure that we have the images that we
expect to have (even if it's an empty set).

R=khushalsagar@chromium.org

Bug: 757049
Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel
Change-Id: Ic544760e175be2bf95d467008cb3b5a59dc32a35
Reviewed-on: https://chromium-review.googlesource.com/703505Reviewed-by: Khushal <khushalsagar@chromium.org>
Commit-Queue: vmpstr <vmpstr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#506909}