Cleanup: Prevent usage of various extension headers when extensions support is not compiled in.

extensions/browser/extension_system.h

 // 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.
 
 #ifndef EXTENSIONS_BROWSER_EXTENSION_SYSTEM_H_
 #define EXTENSIONS_BROWSER_EXTENSION_SYSTEM_H_
 
 #include <string>
 
 #include "base/memory/ref_counted.h"
 #include "components/keyed_service/core/keyed_service.h"
 #include "extensions/common/extension.h"
 
+#if !defined(ENABLE_EXTENSIONS)
+#error "Extensions must be enabled"
+#endif
+
 class ExtensionService;
 
 #if defined(OS_CHROMEOS)
 namespace chromeos {
 class DeviceLocalAccountManagementPolicyProvider;
 }
 #endif  // defined(OS_CHROMEOS)
 
 namespace content {
 class BrowserContext;
@@ skipping 115 matching lines @@
 
   // Get the user script master for declarative scripts, if any.
   virtual DeclarativeUserScriptMaster*
       GetDeclarativeUserScriptMasterByExtension(
           const ExtensionId& extension_id) = 0;
 };
 
 }  // namespace extensions
 
 #endif  // EXTENSIONS_BROWSER_EXTENSION_SYSTEM_H_