Introduce //components/user_prefs, use to eliminate c/b/prefs dependency in Autofill.

components/user_prefs/user_prefs.cc

Index: components/user_prefs/user_prefs.cc
diff --git a/components/user_prefs/user_prefs.cc b/components/user_prefs/user_prefs.cc
new file mode 100644
index 0000000000000000000000000000000000000000..0c920f0db668cb32dcc32c6da3b488b436fd83d1
--- /dev/null
+++ b/components/user_prefs/user_prefs.cc
@@ -0,0 +1,44 @@
+// Copyright 2013 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.
+
+#include "components/user_prefs/user_prefs.h"
+
+#include "base/logging.h"
+#include "base/memory/singleton.h"
+#include "base/prefs/pref_service.h"
+#include "content/public/browser/browser_context.h"
+
+namespace components {
+
+namespace {
+
+void* UserDataKey() {
+  // We just need a unique constant. Use the address of this static member.
+  return reinterpret_cast<void*>(&UserPrefs::Get);
+}
+
+}  // namespace
+
+UserPrefs::UserPrefs(PrefService* prefs) : prefs_(prefs) {
+}
+
+UserPrefs::~UserPrefs() {
+}
+
+// static
+PrefService* UserPrefs::Get(content::BrowserContext* context) {
+  DCHECK(context);
+  return static_cast<UserPrefs*>(
+      context->GetUserData(UserDataKey()))->prefs_;
+}
+
+// static
+void UserPrefs::Set(content::BrowserContext* context, PrefService* prefs) {
+  DCHECK(context);
+  DCHECK(prefs);
+  DCHECK(!context->GetUserData(UserDataKey()));
+  context->SetUserData(UserDataKey(), new UserPrefs(prefs));
+}
+
+}  // namespace components