Introduce a layering in the invalidation component as public and impl

components/invalidation/invalidation_prefs.h

Index: components/invalidation/invalidation_prefs.h
diff --git a/components/invalidation/invalidation_prefs.h b/components/invalidation/invalidation_prefs.h
deleted file mode 100644
index 2fdc5f230e504c02bb95b26474762285a36ed4a0..0000000000000000000000000000000000000000
--- a/components/invalidation/invalidation_prefs.h
+++ /dev/null
@@ -1,15 +0,0 @@
-// 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.
-
-namespace invalidation {
-namespace prefs {
-
-extern const char kInvalidatorClientId[];
-
-extern const char kInvalidatorInvalidationState[];
-
-extern const char kInvalidatorSavedInvalidations[];
-
-}  // namespace prefs
-}  // namespace invalidation