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Although it doesn't flake in Release, the presense of DCHECKs makes
this test fail. Waiting just for the gallery element could read the
button state in the static gallery.html DOM, so wait for the first image
to be fully loaded.

Bug: 857021
Cq-Include-Trybots: luci.chromium.try:closure_compilation
Change-Id: I4e07384b105489410a73894b4a9e0a58bdae287d
Reviewed-on: https://chromium-review.googlesource.com/1125589Reviewed-by: Noel Gordon <noel@chromium.org>
Commit-Queue: Trent Apted <tapted@chromium.org>
Cr-Commit-Position: refs/heads/master@{#572500}