ios: Cleanup class/struct forward declarations

ios/web/navigation/crw_session_entry.h

 // Copyright 2012 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 IOS_WEB_NAVIGATION_CRW_SESSION_ENTRY_H_
 #define IOS_WEB_NAVIGATION_CRW_SESSION_ENTRY_H_
 
 #import <Foundation/Foundation.h>
 #include <stdint.h>
 
 #include <memory>
 
 #include "base/strings/string16.h"
 #include "base/time/time.h"
 #include "ui/base/page_transition_types.h"
 #include "url/gurl.h"
 
 namespace web {
 class NavigationItem;
 class NavigationItemImpl;
-struct Referrer;
 
 // Keys used to serialize web::PageScrollState properties.
 extern NSString* const kSessionEntryPageScrollStateKey;
 extern NSString* const kSessionEntryScrollOffsetXKey;
 extern NSString* const kSessionEntryScrollOffsetYKey;
 extern NSString* const kSessionEntryMinimumZoomScaleKey;
 extern NSString* const kSessionEntryMaximumZoomScaleKey;
 extern NSString* const kSessionEntryZoomScaleKey;
 
 // Keys used to serialize navigation properties.
@@ skipping 39 matching lines @@
 // Pointer to the NavigationItemImpl associated with this CRWSessionEntry.
 @property(nonatomic, readonly) web::NavigationItemImpl* navigationItemImpl;
 
 // Initialize the session entry with the given NavigationItem.
 - (instancetype)initWithNavigationItem:
     (std::unique_ptr<web::NavigationItem>)item;
 
 @end
 
 #endif  // IOS_WEB_NAVIGATION_CRW_SESSION_ENTRY_H_