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In the Holding Space's Screen Capture section, we display screenshots
and screen recordings. At the moment, both previews display as still
images.
In order to differentiate between screenshots and screen recordings, we
add a play icon over the screen recordings.

Screenshot: https://screenshot.googleplex.com/9YVVkvvyAk7Duds

Bug: 1144927
Change-Id: Id1614a867caa73676d24c9ab59289337fea88c63
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2566051
Commit-Queue: Ana Salazar <anasalazar@chromium.org>
Reviewed-by: Toni Baržić <tbarzic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#833785}