ThumbnailImage observers can listen on compressed data.

Exactly what it says on the tin. Compressed JPEG data is now available.
Expected to be used for Mohnstrudel tabstrip.

Change-Id: I523ad8a65a38b7a6c66bc462c8f78ca7d5012318
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1817583
Reviewed-by: Taylor Bergquist <tbergquist@chromium.org>
Commit-Queue: Dana Fried <dfried@chromium.org>
Cr-Commit-Position: refs/heads/master@{#700445}