[Offline Pages] Add attribution, snippet and favicon to offline pages database

This change adds new fields to the pages and thumbnails tables.

Bug: 841516
Change-Id: I79144f0debb3d4ffdc067ee9299b5bb6af4bfe9c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1549687
Commit-Queue: Ian Wells <iwells@chromium.org>
Reviewed-by: Dan H <harringtond@google.com>
Cr-Commit-Position: refs/heads/master@{#647044}