[EoS] Adds an IPH prompt for the Explore tile on the NTP.

This hooks up the existing IPH in a new place when the
Most Likely variant is enabled.

Bug: 966024
Change-Id: I5808f13f1d7047beb7dc3328fee5d0941b4d2d54
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1637454
Commit-Queue: Justin DeWitt <dewittj@chromium.org>
Reviewed-by: Theresa <twellington@chromium.org>
Reviewed-by: Cathy Li <chili@chromium.org>
Cr-Commit-Position: refs/heads/master@{#665741}