Add CWVUIDelegate with method to customize context menu.

ios/web_view/public/cwv_ui_delegate.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_UI_DELEGATE_H_
+#define IOS_WEB_VIEW_PUBLIC_CWV_UI_DELEGATE_H_
+
+#import <CoreGraphics/CoreGraphics.h>
+#import <UIKit/UIKit.h>
+
+@class CWVHTMLElement;
+@class CWVWebView;
+
+// UI delegate interface for a CWVWebView.  Embedders can implement the
+// functions in order to customize library behavior.
+@protocol CWVUIDelegate<NSObject>
+
+@optional
+// Instructs the delegate to present context menu in response to user’s long
+// press gesture at |location| in |view| coordinate space. |element| is an HTML
+// element which received the gesture.
+- (void)webView:(CWVWebView*)webView
+    runContextMenuWithTitle:(NSString*)menuTitle
+             forHTMLElement:(CWVHTMLElement*)element
+                     inView:(UIView*)view
+        userGestureLocation:(CGPoint)location;
+
+@end
+
+#endif  // IOS_WEB_VIEW_PUBLIC_CWV_UI_DELEGATE_H_