Add field trial config for QueryTiles NTP stable experiment

internal finch config: cl/346202862

Bug: 1127441
Change-Id: I7428aa78cedaf4c0990e909dd8d3809254a36f3e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2578109


Reviewed-by: Caitlin Fischer <caitlinfischer@google.com>
Reviewed-by: Min Qin <qinmin@chromium.org>
Commit-Queue: Min Qin <qinmin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#834807}