Android: Do not build any extensions code except for extensions_constants.

extensions/common/extension.h

 // Copyright (c) 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 EXTENSIONS_COMMON_EXTENSION_H_
 #define EXTENSIONS_COMMON_EXTENSION_H_
 
 #include <algorithm>
 #include <iosfwd>
 #include <map>
@@ skipping 10 matching lines @@
 #include "base/synchronization/lock.h"
 #include "base/threading/thread_checker.h"
 #include "extensions/common/extension_resource.h"
 #include "extensions/common/install_warning.h"
 #include "extensions/common/manifest.h"
 #include "extensions/common/url_pattern_set.h"
 #include "ui/base/accelerators/accelerator.h"
 #include "ui/gfx/size.h"
 #include "url/gurl.h"
 
+#if !defined(ENABLE_EXTENSIONS)
+#error "Extensions must be enabled"
+#endif
+
 namespace base {
 class DictionaryValue;
 class Version;
 }
 
 namespace extensions {
 class PermissionSet;
 class PermissionsData;
 class PermissionsParser;
 
@@ skipping 523 matching lines @@
 
   UpdatedExtensionPermissionsInfo(
       const Extension* extension,
       const PermissionSet* permissions,
       Reason reason);
 };
 
 }  // namespace extensions
 
 #endif  // EXTENSIONS_COMMON_EXTENSION_H_