Add public API for handling Javascript alerts and prompt.

ios/web/public/web_state/web_state_delegate.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_H_
 #define IOS_WEB_PUBLIC_WEB_STATE_WEB_STATE_DELEGATE_H_
 
 #include <set>
 
 #include "base/callback.h"
 
 namespace net {
 class SSLInfo;
 }
 
 namespace web {
 
 struct ContextMenuParams;
+class JavaScriptDialogPresenter;
 struct SSLStatus;
 class WebState;
 
 // Objects implement this interface to get notified about changes in the
 // WebState and to provide necessary functionality.
 class WebStateDelegate {
  public:
   WebStateDelegate();
 
   // Notifies the delegate that the page has made some progress loading.
   // |progress| is a value between 0.0 (nothing loaded) to 1.0 (page fully
   // loaded).
   virtual void LoadProgressChanged(WebState* source, double progress);
 
   // Notifies the delegate that the user triggered the context menu with the
   // given |ContextMenuParams|. Returns true if the context menu operation was
   // handled by the delegate.
   virtual bool HandleContextMenu(WebState* source,
                                  const ContextMenuParams& params);
 
+  // Returns a pointer to a service to manage dialogs. May return nullptr in
+  // which case dialogs aren't shown.
+  // TODO(crbug.com/622084): Find better place for this method.
+  virtual JavaScriptDialogPresenter* GetJavaScriptDialogPresenter(
+      WebState* source);
+
  protected:
   virtual ~WebStateDelegate();
 
  private:
   friend class WebStateImpl;
 
   // Called when |this| becomes the WebStateDelegate for |source|.
   void Attach(WebState* source);
 
   // Called when |this| is no longer the WebStateDelegate for |source|.
   void Detach(WebState* source);
 
   // The WebStates for which |this| is currently a delegate.
   std::set<WebState*> attached_states_;
 };
 
 }  // namespace web
 
 #endif  // IOS_WEB_PUBLIC_WEB_STATE_WEB_STATE_DELEGATE_H_