Add public API for handling Javascript alerts and prompt.

ios/web/public/web_state/web_state_delegate_bridge.h

 // Copyright 2016 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_PUBLIC_WEB_STATE_WEB_STATE_DELEGATE_BRIDGE_H_
 #define IOS_WEB_PUBLIC_WEB_STATE_WEB_STATE_DELEGATE_BRIDGE_H_
 
 #import <Foundation/Foundation.h>
 
 #import "base/ios/weak_nsobject.h"
 #include "ios/web/public/web_state/web_state_delegate.h"
 
 // Objective-C interface for web::WebStateDelegate.
 @protocol CRWWebStateDelegate<NSObject>
 @optional
 
 // Called when the page has made some progress loading. |progress| is a value
 // between 0.0 (nothing loaded) to 1.0 (page fully loaded).
 - (void)webState:(web::WebState*)webState didChangeProgress:(double)progress;
 
 // Called when the user triggers the context menu with the given
 // |ContextMenuParams|. Returns YES if the context menu operation was
 // handled by the delegate. If this method is not implemented, the system
 // context menu will be displayed.
 - (BOOL)webState:(web::WebState*)webState
     handleContextMenu:(const web::ContextMenuParams&)params;
 
+// Returns a pointer to a service to manage dialogs. May return null in which
+// case dialogs aren't shown.
+- (web::JavaScriptDialogPresenter*)javaScriptDialogPresenterForWebState:
+    (web::WebState*)webState;
+
 @end
 
 namespace web {
 
 // Adapter to use an id<CRWWebStateDelegate> as a web::WebStateDelegate.
 class WebStateDelegateBridge : public web::WebStateDelegate {
  public:
   explicit WebStateDelegateBridge(id<CRWWebStateDelegate> delegate);
   ~WebStateDelegateBridge() override;
 
   // web::WebStateDelegate methods.
   void LoadProgressChanged(WebState* source, double progress) override;
   bool HandleContextMenu(WebState* source,
                          const ContextMenuParams& params) override;
+  JavaScriptDialogPresenter* GetJavaScriptDialogPresenter(
+      WebState* source) override;
 
  private:
   // CRWWebStateDelegate which receives forwarded calls.
   base::WeakNSProtocol<id<CRWWebStateDelegate>> delegate_;
   DISALLOW_COPY_AND_ASSIGN(WebStateDelegateBridge);
 };
 
 }  // web
 
 #endif  // IOS_WEB_PUBLIC_WEB_STATE_WEB_STATE_DELEGATE_BRIDGE_H_