[NTP] Mojoify chrome://snippets-internals backend

Create/implement a mojo interface for snippets-internals.

Bug: 830931
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I48978cab1c26e88d588f6f56e6cae80cbe13826b
Reviewed-on: https://chromium-review.googlesource.com/1033807
Commit-Queue: Brandon Wylie <wylieb@chromium.org>
Reviewed-by: calamity <calamity@chromium.org>
Reviewed-by: Patrick Noland <pnoland@chromium.org>
Reviewed-by: Filip Gorski <fgorski@chromium.org>
Cr-Commit-Position: refs/heads/master@{#559227}
15 files changed