Add CRWWebViewScrollViewProxy.dragging which will be used in

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 75 matching lines @@
 }
 
 - (BOOL)bounces {
   return [_scrollView bounces];
 }
 
 - (void)setBounces:(BOOL)bounces {
   [_scrollView setBounces:bounces];
 }
 
+- (BOOL)isDragging {
+  return [_scrollView isDragging];
+}
+
 - (BOOL)isZooming {
   return [_scrollView isZooming];
 }
 
 - (CGFloat)zoomScale {
   return [_scrollView zoomScale];
 }
 
 - (void)setContentOffset:(CGPoint)contentOffset {
   [_scrollView setContentOffset:contentOffset];
@@ skipping 118 matching lines @@
 - (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