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

components/user_prefs/user_prefs_export.h

Index: components/user_prefs/user_prefs_export.h
diff --git a/components/user_prefs/user_prefs_export.h b/components/user_prefs/user_prefs_export.h
new file mode 100644
index 0000000000000000000000000000000000000000..3d20222a2b9e40aa71ead58304767b34176da471
--- /dev/null
+++ b/components/user_prefs/user_prefs_export.h
@@ -0,0 +1,29 @@
+// 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.
+
+#ifndef COMPONENTS_USER_PREFS_USER_PREFS_EXPORT_H_
+#define COMPONENTS_USER_PREFS_USER_PREFS_EXPORT_H_
+
+#if defined(COMPONENT_BUILD)
+#if defined(WIN32)
+
+#if defined(USER_PREFS_IMPLEMENTATION)
+#define USER_PREFS_EXPORT __declspec(dllexport)
+#else
+#define USER_PREFS_EXPORT __declspec(dllimport)
+#endif  // defined(BASE_PREFS_IMPLEMENTATION)
+
+#else  // defined(WIN32)
+#if defined(USER_PREFS_IMPLEMENTATION)
+#define USER_PREFS_EXPORT __attribute__((visibility("default")))
+#else
+#define USER_PREFS_EXPORT
+#endif
+#endif
+
+#else  // defined(COMPONENT_BUILD)
+#define USER_PREFS_EXPORT
+#endif
+
+#endif  // COMPONENTS_USER_PREFS_USER_PREFS_EXPORT_H_