extensions: Cleanup class/struct fwd declarations

extensions/browser/extension_navigation_throttle.h

 // Copyright 2016 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_NAVIGATION_THROTTLE_H_
 #define EXTENSIONS_BROWSER_EXTENSION_NAVIGATION_THROTTLE_H_
 
 #include "base/macros.h"
 #include "content/public/browser/navigation_throttle.h"
 
 namespace content {
 class NavigationHandle;
 }
 
 namespace extensions {
 
-class NavigationParams;
-
 // This class allows the extensions subsystem to have control over navigations
 // and optionally cancel/block them. This is a UI thread class.
 class ExtensionNavigationThrottle : public content::NavigationThrottle {
  public:
   explicit ExtensionNavigationThrottle(
       content::NavigationHandle* navigation_handle);
   ~ExtensionNavigationThrottle() override;
 
   // content::NavigationThrottle implementation:
   ThrottleCheckResult WillStartRequest() override;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(ExtensionNavigationThrottle);
 };
 
 }  // namespace extensions
 
 #endif  // EXTENSIONS_BROWSER_EXTENSION_NAVIGATION_THROTTLE_H_