[NTP] Implement add/delete custom links

Add API to support adding/deleting custom links. Once custom links are
enabled, MV tiles are "frozen" and converted to custom links.

Design doc: go/chrome-ntp-custom-links

Bug: 856394
Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: Ib3255d7253fa9964b6f7845edf5cba4be2fa3db9
Reviewed-on: https://chromium-review.googlesource.com/1124104
Commit-Queue: Kristi Park <kristipark@chromium.org>
Reviewed-by: Marc Treib <treib@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Reviewed-by: Mathieu Perreault <mathp@chromium.org>
Reviewed-by: Dominic Battré <battre@chromium.org>
Cr-Commit-Position: refs/heads/master@{#575952}
25 files changed