extensions: Cleanup class/struct fwd declarations

extensions/browser/guest_view/mime_handler_view/mime_handler_stream_manager.h

 // Copyright 2015 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_GUEST_VIEW_MIME_HANDLER_VIEW_MIME_HANDLER_STREAM_MANAGER_H_
 #define EXTENSIONS_BROWSER_GUEST_VIEW_MIME_HANDLER_VIEW_MIME_HANDLER_STREAM_MANAGER_H_
 
 #include <map>
 #include <set>
 #include <string>
 
 #include "base/memory/ref_counted.h"
 #include "base/scoped_observer.h"
 #include "components/keyed_service/core/keyed_service.h"
 #include "extensions/browser/extension_registry_observer.h"
 
 namespace content {
 class BrowserContext;
-class WebContents;
 }
 
 namespace extensions {
 class Extension;
 class StreamContainer;
 
 // A container for streams that have not yet been claimed by a
 // MimeHandlerViewGuest. If the embedding RenderFrameHost is closed or navigates
 // away from the resource being streamed, the stream is aborted. This is
 // BrowserContext-keyed because mime handlers are extensions, which are
@@ skipping 38 matching lines @@
   // EmbedderObserver until it is removed.
   std::map<std::string, std::unique_ptr<EmbedderObserver>> embedder_observers_;
 
   ScopedObserver<ExtensionRegistry, ExtensionRegistryObserver>
       extension_registry_observer_;
 };
 
 }  // namespace extensions
 
 #endif  // EXTENSIONS_BROWSER_GUEST_VIEW_MIME_HANDLER_VIEW_MIME_HANDLER_STREAM_MANAGER_H_