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The close button is not accessible by ChromeVox since
it is not drawn. Therefore, we should switch between a
blank image and the icon.

BUG=994838

Change-Id: I008b08b8620c20504806958eedeb8e26a112c0ce
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1758718
Commit-Queue: Becca Hughes <beccahughes@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#688226}