Plumb favicon URL, snippet, and attribution from Feed to prefetch store

The offline prefetch DB already has fields for an article's favicon URL,
snippet, and attribution text in its prefetch_items table. This change makes
these things available from feed suggestions when new items are added to the
store.

Bug: 841516
Change-Id: Ie8a88007d4fb115cedce174da3fb0f0e5c806b81
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1526794
Commit-Queue: Ian Wells <iwells@chromium.org>
Reviewed-by: Carlos Knippschild <carlosk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#643081}
5 files changed