Add a tool to generate preload data in DAFSA format and
add a MediaEngagementPreloadedList class to read it.

BUG=787464

TBR=dpranke@chromium.org

Change-Id: If0d99a2e1091d0b23699f2173d8146582173ee54
Reviewed-on: https://chromium-review.googlesource.com/803994
Commit-Queue: Becca Hughes <beccahughes@chromium.org>
Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
Cr-Commit-Position: refs/heads/master@{#522068}