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Add an IntersectionObserver to the thumbnail bar. When the thumbnails
scroll into the observable area, fire an event to request the thumbnail.

The requests are not handled yet.

Bug: 652400
Change-Id: I6a3dc3ce59304dca20b2d8d828a97e98a3388b92
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2430443
Commit-Queue: Daniel Hosseinian <dhoss@chromium.org>
Reviewed-by: dpapad <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#810879}