[Feed Internals] Add 4 new properties to page

This internals page is used to surface debugging information useful to
developers of the NTP Interest Feed.

Adds 4 additional simple properties to the debugging page:
- whether the Feed is visible
- whether the Feed is allowed (by policy)
- whether prefetching for offline use is enabled
- the reason for the last fetch

https://screenshot.googleplex.com/efYU09sWvNP

Bug: 913126
Change-Id: Ib9cad2a6fca27b292f4cb5198e1298fc72ad13ae
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1512267
Commit-Queue: Natalie Chouinard <chouinard@chromium.org>
Reviewed-by: Sky Malice <skym@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#639691}
7 files changed