Local NTP: Expose "MV available" to the page, don't load tiles until it is

This works around a race condition between loading the page and the IPC
message that contains the MV items.
Before this CL, if the page load won the race, then it would assume there
are no tiles and load incorrect metrics. This CL exposes "did we ever
receive MV tiles" to the page, so it can delay loading them while they're
not available anyway.

This is very much working around the symptoms rather than fixing the root
cause. Unfortunately I have no idea how to really fix the issue.

Bug: 794942, 793818
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I091b71d17d36e28b3f53ceebef6829ef23d82b3b
Reviewed-on: https://chromium-review.googlesource.com/829254


Reviewed-by: Chris Pickel <sfiera@chromium.org>
Commit-Queue: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#524415}