Network Service: Fix invalid use of std::move in network_hints_message_filter.cc

This CL fixes invalid use of std::move() in network_hints_message_filter.cc
by replacing it with std::unique_ptr::reset().

Bug: 981366
Change-Id: Ib5168dbe68d9e74417e0fb27474ec891605e5357
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1693947
Reviewed-by: Eric Roman <eroman@chromium.org>
Commit-Queue: Jun Cai <juncai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#675880}