GitLab

Enhance thumbnail-related test coverage

- All thumbnails shown can be released after GTS is gone.
- No thumbnails are loaded before GTS becomes visible.

Bug: 983170
Change-Id: Ib7b38909625033c82213f728b6d2e9d4a33e7b53
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1868844
Commit-Queue: Wei-Yin Chen (陳威尹) <wychen@chromium.org>
Reviewed-by: Yusuf Ozuysal <yusufo@chromium.org>
Reviewed-by: Yue Zhang <yuezhanggg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#708774}