Download thumbnails when GeneratePageBundle is requested

This change adds a new trigger to the downloading of thumbnails for
prefetched offline articles right after GeneratePageBundle is
requested. This is in addition to the later attempt made when a
prefetched article is successfully downloaded and aims at improving the
availability of article thumbnails.

Bug: 794828
Change-Id: Ic857ab91f4f2befb327e91300005c59c10bc417d
Reviewed-on: https://chromium-review.googlesource.com/1047991
Commit-Queue: Carlos Knippschild <[email protected]>
Reviewed-by: Dmitry Titov <[email protected]>
Cr-Commit-Position: refs/heads/master@{#557069}
19 files changed