Add public API for handling Javascript alerts and prompt.

ios/web/web_state/web_state_delegate.mm

 // 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.
 
 #import "ios/web/public/web_state/web_state_delegate.h"
 
 #import "ios/web/public/web_state/web_state.h"
 
 namespace web {
 
 WebStateDelegate::WebStateDelegate() {}
 
 WebStateDelegate::~WebStateDelegate() {
   while (!attached_states_.empty()) {
     WebState* web_state = *attached_states_.begin();
     web_state->SetDelegate(nullptr);
   }
   DCHECK(attached_states_.empty());
 }
 
 void WebStateDelegate::LoadProgressChanged(WebState*, double) {}
 
 bool WebStateDelegate::HandleContextMenu(WebState*, const ContextMenuParams&) {
   return false;
 }
 
+DialogPresenter* WebStateDelegate::GetDialogPresenter(WebState*) {
+  return nullptr;
+}
+
 void WebStateDelegate::Attach(WebState* source) {
   DCHECK(attached_states_.find(source) == attached_states_.end());
   attached_states_.insert(source);
 }
 
 void WebStateDelegate::Detach(WebState* source) {
   DCHECK(attached_states_.find(source) != attached_states_.end());
   attached_states_.erase(source);
 }
 
 }  // web