NTP: VariationParamsManager resetting works, so make it a member.

This CL makes use of the resetting function VariationParamsManager.
This should work now and therefore it is sufficient to hold the manager
as a simple member.

BUG=672010

Review-Url: https://codereview.chromium.org/2592663003
Cr-Commit-Position: refs/heads/master@{#439839}