Fix: use forward declaration in public header file of ChromeWebView

Change-Id: Ia1980395380f62708b89fff2f9d5208165d58e95
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2006830Reviewed-by: Hiroshi Ichikawa <ichikawa@chromium.org>
Commit-Queue: Jiajun Ou <garzonou@google.com>
Cr-Commit-Position: refs/heads/master@{#732741}

ios/web_view/public/cwv_back_forward_list.h

@@ -9,10 +9,9 @@
 
 #import "cwv_export.h"
 
-#import "ios/web_view/public/cwv_back_forward_list_item.h"
-
 NS_ASSUME_NONNULL_BEGIN
 
+@class CWVBackForwardListItem;
 @class CWVWebView;
 
 // This just behaves like a NSArray<CWVBackForwardListItem*>.