components: Make user_prefs a source_set.

components/user_prefs/BUILD.gn

 # Copyright 2014 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("user_prefs") {
+source_set("user_prefs") {
   sources = [
     "user_prefs.cc",
     "user_prefs.h",
-    "user_prefs_export.h",
   ]
 
-  defines = [ "USER_PREFS_IMPLEMENTATION" ]
-
   deps = [
     "//base",
     "//base:prefs",
   ]
 }