GitLab

Ensures subscribers know when a thumbnail is cleared, and avoids
sending them stale thumbnails from previous requests.

Fixed: 1163121
Change-Id: I31d881808fb2a6bb14ae20b7d5b2d67545b585c4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2628031Reviewed-by: Dana Fried <dfried@chromium.org>
Commit-Queue: Collin Baker <collinbaker@chromium.org>
Cr-Commit-Position: refs/heads/master@{#844166}