Add -[CRWWebViewScrollViewObserver webViewScrollViewWillBeginZooming...]

It will be used in CWVScrollView.

Review-Url: https://codereview.chromium.org/2876693006
Cr-Commit-Position: refs/heads/master@{#471178}

ios/web/public/web_state/ui/crw_web_view_scroll_view_proxy.h

@@ -80,6 +80,12 @@
     (CRWWebViewScrollViewProxy*)webViewScrollViewProxy;
 - (void)webViewScrollViewDidResetContentSize:
     (CRWWebViewScrollViewProxy*)webViewScrollViewProxy;
+
+// The equivalent in UIScrollViewDelegate also takes a parameter (UIView*)view,
+// but CRWWebViewScrollViewObserver doesn't expose it for flexibility of future
+// implementation.
+- (void)webViewScrollViewWillBeginZooming:
+    (CRWWebViewScrollViewProxy*)webViewScrollViewProxy;
 @end
 
 // A protocol to be implemented by objects to listen for changes to the

ios/web/web_state/ui/crw_web_view_scroll_view_proxy.mm

@@ -213,6 +213,12 @@
   [_observers webViewScrollViewDidZoom:self];
 }
 
+- (void)scrollViewWillBeginZooming:(UIScrollView*)scrollView
+                          withView:(UIView*)view {
+  DCHECK_EQ(_scrollView, scrollView);
+  [_observers webViewScrollViewWillBeginZooming:self];
+}
+
 #pragma mark -
 
 + (NSArray*)scrollViewObserverKeyPaths {