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}
diff --git a/chrome/browser/search/instant_service_unittest.cc b/chrome/browser/search/instant_service_unittest.cc
index 66345ae..3f5dc50 100644
--- a/chrome/browser/search/instant_service_unittest.cc
+++ b/chrome/browser/search/instant_service_unittest.cc
@@ -20,7 +20,7 @@
 #include "chrome/common/url_constants.h"
 #include "components/ntp_tiles/constants.h"
 #include "components/ntp_tiles/ntp_tile.h"
-#include "components/ntp_tiles/pref_names.cc"
+#include "components/ntp_tiles/pref_names.h"
 #include "components/ntp_tiles/section_type.h"
 #include "components/sync_preferences/testing_pref_service_syncable.h"
 #include "testing/gmock/include/gmock/gmock.h"