Add -[CRWWebViewScrollViewObserver webViewScrollViewWillBeginZooming...]

ios/web/web_state/ui/crw_web_view_scroll_view_proxy.mm

 // Copyright 2014 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.
 
 #import "ios/web/public/web_state/ui/crw_web_view_scroll_view_proxy.h"
 
 #import <objc/runtime.h>
 
 #include "base/auto_reset.h"
 #import "base/ios/crb_protocol_observers.h"
@@ skipping 191 matching lines @@
     }
   }];
   return shouldScrollToTop;
 }
 
 - (void)scrollViewDidZoom:(UIScrollView*)scrollView {
   DCHECK_EQ(_scrollView, scrollView);
   [_observers webViewScrollViewDidZoom:self];
 }
 
+- (void)scrollViewWillBeginZooming:(UIScrollView*)scrollView
+                          withView:(UIView*)view {
+  DCHECK_EQ(_scrollView, scrollView);
+  [_observers webViewScrollViewWillBeginZooming:self];
+}
+
 #pragma mark -
 
 + (NSArray*)scrollViewObserverKeyPaths {
   return @[ @"contentSize" ];
 }
 
 - (void)startObservingScrollView:(UIScrollView*)scrollView {
   for (NSString* keyPath in [[self class] scrollViewObserverKeyPaths])
     [scrollView addObserver:self forKeyPath:keyPath options:0 context:nil];
 }
 
 - (void)stopObservingScrollView:(UIScrollView*)scrollView {
   for (NSString* keyPath in [[self class] scrollViewObserverKeyPaths])
     [scrollView removeObserver:self forKeyPath:keyPath];
 }
 
 - (void)observeValueForKeyPath:(NSString*)keyPath
                       ofObject:(id)object
                         change:(NSDictionary*)change
                        context:(void*)context {
   DCHECK_EQ(object, _scrollView);
   if ([keyPath isEqualToString:@"contentSize"])
     [_observers webViewScrollViewDidResetContentSize:self];
 }
 
 @end