Add CWVUIDelegate with method to customize context menu.

ios/web_view/public/cwv_html_element.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_HTML_ELEMENT_H_
+#define IOS_WEB_VIEW_PUBLIC_CWV_HTML_ELEMENT_H_
+
+#import <Foundation/Foundation.h>
+
+// Encapsulates information about HTML element. Used in
+// delegate methods.
+@interface CWVHTMLElement : NSObject
+
+// |href| property of an HTML element.
+@property(nullable, copy, readonly) NSURL* hyperlink;
+// |src| property of an HTML element.
+@property(nullable, copy, readonly) NSURL* mediaSource;
+// |innerText| property of an HTML element.
+@property(nullable, copy, readonly) NSString* text;
+@end
+
+#endif  // IOS_WEB_VIEW_PUBLIC_CWV_HTML_ELEMENT_H_