[NTP] Cap search suggestion impressions

The parameters for capping suggestions impressions are provided as part
of the update proto. Read and update the local pref on each fetch. Use
these prefs to determine if the impression cap has been reach or if
fetching is frozen due to an empty response.

Bug: 904565
Change-Id: Ib5539a99f18e9da2ac1223ecc7aff65cb909bca8
Reviewed-on: https://chromium-review.googlesource.com/c/1395188
Commit-Queue: Kyle Milka <kmilka@chromium.org>
Reviewed-by: Kristi Park <kristipark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#624994}
11 files changed