extensions: Cleanup class/struct fwd declarations

extensions/browser/external_provider_interface.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 EXTENSIONS_BROWSER_EXTERNAL_PROVIDER_INTERFACE_H_
 #define EXTENSIONS_BROWSER_EXTERNAL_PROVIDER_INTERFACE_H_
 
 #include <memory>
 #include <vector>
 
 #include "extensions/common/manifest.h"
 
-class GURL;
-
 namespace base {
 class Version;
 }
 
 namespace extensions {
 
 struct ExternalInstallInfoFile;
 struct ExternalInstallInfoUpdateUrl;
 
 // This class is an abstract class for implementing external extensions
@@ skipping 72 matching lines @@
   // from its source.
   virtual bool IsReady() const = 0;
 };
 
 using ProviderCollection =
     std::vector<std::unique_ptr<ExternalProviderInterface>>;
 
 }  // namespace extensions
 
 #endif  // EXTENSIONS_BROWSER_EXTERNAL_PROVIDER_INTERFACE_H_