Add missing ExtensionHost forward declaration to ProcessManagerObserver.

extensions/browser/process_manager_observer.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_PROCESS_MANAGER_OBSERVER_H_
 #define EXTENSIONS_BROWSER_PROCESS_MANAGER_OBSERVER_H_
 
 #include <string>
 
 namespace extensions {
 class Extension;
+class ExtensionHost;
 
 class ProcessManagerObserver {
  public:
   // Called immediately after an extension background host is started.
   virtual void OnBackgroundHostStartup(const Extension* extension) {}
 
   // Called immediately after an ExtensionHost for an extension with a lazy
   // background page is created.
   virtual void OnBackgroundHostCreated(ExtensionHost* host) {}
 
   // Called immediately after the extension background host is destroyed.
   virtual void OnBackgroundHostClose(const std::string& extension_id) {}
 };
 
 }  // namespace extensions
 
 #endif  // EXTENSIONS_BROWSER_PROCESS_MANAGER_OBSERVER_H_