Convert some GN test support targets to static libs.

components/prefs/BUILD.gn

 # Copyright 2015 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 component("prefs") {
   sources = [
     "default_pref_store.cc",
     "default_pref_store.h",
     "in_memory_pref_store.cc",
     "in_memory_pref_store.h",
@@ skipping 37 matching lines @@
     ]
   }
 
   defines = [ "COMPONENTS_PREFS_IMPLEMENTATION" ]
 
   deps = [
     "//base",
   ]
 }
 
-source_set("test_support") {
+static_library("test_support") {
   testonly = true
   sources = [
     "mock_pref_change_callback.cc",
     "mock_pref_change_callback.h",
     "pref_store_observer_mock.cc",
     "pref_store_observer_mock.h",
     "testing_pref_service.cc",
     "testing_pref_service.h",
     "testing_pref_store.cc",
     "testing_pref_store.h",
@@ skipping 26 matching lines @@
   ]
 
   deps = [
     ":test_support",
     "//base",
     "//base/test:test_support",
     "//testing/gmock",
     "//testing/gtest",
   ]
 }