Include the header instead of cc file

A unit test includes prefs_names.cc which in some builds
caused duplicate symbol errors in linking.

(I get a feeling we need a presubmit check for this because
it has happened several times recently)

TBR=gayane@chromium.org

Bug: 931806
Change-Id: I942ba67f7570feed59b32694a55402db169e109c
Reviewed-on: https://chromium-review.googlesource.com/c/1494884
Reviewed-by: Daniel Bratell <bratell@opera.com>
Commit-Queue: Daniel Bratell <bratell@opera.com>
Cr-Commit-Position: refs/heads/master@{#636444}