Build a few more extensions files only when extensions are enabled.

chrome/common/extensions/features/feature_channel.h

 // 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 CHROME_COMMON_EXTENSIONS_FEATURES_FEATURE_CHANNEL_H_
 #define CHROME_COMMON_EXTENSIONS_FEATURES_FEATURE_CHANNEL_H_
 
 #include "chrome/common/chrome_version_info.h"
 
 namespace extensions {
 
 // Gets the current channel as seen by the Feature system.
 chrome::VersionInfo::Channel GetCurrentChannel();
 
 // Sets the current channel as seen by the Feature system. In the browser
 // process this should be chrome::VersionInfo::GetChannel(), and in the
 // renderer this will need to come from an IPC.
 void SetCurrentChannel(chrome::VersionInfo::Channel channel);
 
 // Gets the default channel as seen by the Feature system.
 chrome::VersionInfo::Channel GetDefaultChannel();
 
 // Scoped channel setter. Use for tests.
 class ScopedCurrentChannel {
  public:
-  explicit ScopedCurrentChannel(chrome::VersionInfo::Channel channel)
-      : original_channel_(chrome::VersionInfo::CHANNEL_UNKNOWN) {
-    original_channel_ = GetCurrentChannel();
-    SetCurrentChannel(channel);
-  }
-
-  ~ScopedCurrentChannel() {
-    SetCurrentChannel(original_channel_);
-  }
+  explicit ScopedCurrentChannel(chrome::VersionInfo::Channel channel);
+  ~ScopedCurrentChannel();
 
  private:
   chrome::VersionInfo::Channel original_channel_;
+
+  DISALLOW_COPY_AND_ASSIGN(ScopedCurrentChannel);
 };
 
 }  // namespace extensions
 
 #endif  // CHROME_COMMON_EXTENSIONS_FEATURES_FEATURE_CHANNEL_H_