Add CWVUIDelegate with method to customize context menu.

ios/web_view/internal/cwv_html_element_internal.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_INTERNAL_CWV_HTML_ELEMENT_INTERNAL_H_
+#define IOS_WEB_VIEW_INTERNAL_CWV_HTML_ELEMENT_INTERNAL_H_
+
+#import "ios/web_view/public/cwv_html_element.h"
+
+@interface CWVHTMLElement ()
+
+- (nonnull instancetype)init NS_UNAVAILABLE;
+
+- (nonnull instancetype)initWithHyperlink:(nullable NSURL*)hyperlink
+                              mediaSource:(nullable NSURL*)mediaSource
+                                     text:(nullable NSString*)text
+    NS_DESIGNATED_INITIALIZER;
+
+@end
+
+#endif  // IOS_WEB_VIEW_INTERNAL_CWV_HTML_ELEMENT_INTERNAL_H_