GN: Add missing dep from //base:prefs_test_support on //testing/gtest.

gtest.h is included pref_store_observer_mock.cc and testing_pref_service.cc.
Found using 'gn check'.

Review URL: https://codereview.chromium.org/435983003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@287540 0039d316-1c4b-4281-b951-d872f2087c98

base/BUILD.gn

@@ -1021,6 +1021,7 @@ source_set("prefs_test_support") {
     ":base",
     ":prefs",
     "//testing/gmock",
+    "//testing/gtest",
   ]
 }