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Test was affected by paper-icon-button migration. This selects the
correct element to check for the hidden attribute.

Bug: 821759
Change-Id: I1bc67c41e9e9293c5f06ef0120fdde85beb959d6
Reviewed-on: https://chromium-review.googlesource.com/978470Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Commit-Queue: Hector Carmona <hcarmona@chromium.org>
Cr-Commit-Position: refs/heads/master@{#545566}