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Previously, user has no way to know what does the taken screenshot looks
like before opening the image. With this CL, user can know that because
the preview image is shown in the notification.
This improvement is included in MD notification mock (internal:
go/fuwow), but should also be useful in the existing style notification.

BUG=746194
TEST=manual

Change-Id: I566b15209b5a7c18b8c706117896c21dde728088
Reviewed-on: https://chromium-review.googlesource.com/580178
Commit-Queue: Tetsui Ohkubo <tetsui@chromium.org>
Reviewed-by: Dan Erat <derat@chromium.org>
Reviewed-by: Naoki Fukino <fukino@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491907}