Add more methods to CWVScrollView and CWVScrollViewDelegate.

ios/web_view/public/cwv_scroll_view_delegate.h

 // Copyright 2017 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_VIEW_PUBLIC_CWV_SCROLL_VIEW_DELEGATE_H_
 #define IOS_WEB_VIEW_PUBLIC_CWV_SCROLL_VIEW_DELEGATE_H_
 
 #import <CoreGraphics/CoreGraphics.h>
 
 @class CWVScrollView;
 
 // Delegete for CWVScrollView.
 //
 // These methods are forwarded from the internal UIScrollViewDelegate. Please
 // see the <UIKit/UIScrollViewDelegate.h> documentation for details about the
 // following methods.
 CWV_EXPORT
 @protocol CWVScrollViewDelegate<NSObject>
 @optional
 - (void)scrollViewWillBeginDragging:(CWVScrollView*)scrollView;
 - (void)scrollViewWillEndDragging:(CWVScrollView*)scrollView
                      withVelocity:(CGPoint)velocity
               targetContentOffset:(inout CGPoint*)targetContentOffset;
 - (void)scrollViewDidScroll:(CWVScrollView*)scrollView;
 - (void)scrollViewDidEndDecelerating:(CWVScrollView*)scrollView;
+
+// The equivalent in UIScrollViewDelegate also takes a parameter (UIView*)view,
+// but CWVScrollViewDelegate doesn't expose it for flexibility of future
+// implementation.
+- (void)scrollViewWillBeginZooming:(CWVScrollView*)webViewScrollViewProxy;
 @end
 
 #endif  // IOS_WEB_VIEW_PUBLIC_CWV_SCROLL_VIEW_DELEGATE_H_