Add CWVUIDelegate with method to customize context menu.

ios/web_view/public/cwv_web_view.h

Index: ios/web_view/public/cwv_web_view.h
diff --git a/ios/web_view/public/cwv_web_view.h b/ios/web_view/public/cwv_web_view.h
index 09a916c1c55ebfd87349d048b45a4ac63b4891fd..0acd82176c1df6d474ad4de5833c6aa86fd7f009 100644
--- a/ios/web_view/public/cwv_web_view.h
+++ b/ios/web_view/public/cwv_web_view.h
@@ -7,6 +7,7 @@
 #import <UIKit/UIKit.h>
 
 @class CWVWebViewConfiguration;
+@protocol CWVUIDelegate;
 @protocol CWVWebViewDelegate;
 
 // A web view component (like WKWebView) which uses iOS Chromium's web view
@@ -24,6 +25,9 @@
 // This web view's delegate.
 @property(nonatomic, weak) id<CWVWebViewDelegate> delegate;
 
+// This web view's UI delegate
+@property(nonatomic, weak) id<CWVUIDelegate> UIDelegate;
+
 // Whether or not this web view can go backwards or forwards.
 @property(nonatomic, readonly) BOOL canGoBack;
 @property(nonatomic, readonly) BOOL canGoForward;