Add public API for handling Javascript alerts and prompt.

ios/web/web_state/web_state_delegate_bridge.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_bridge.h"
 
 #include "base/logging.h"
 #import "ios/web/public/web_state/context_menu_params.h"
 
 namespace web {
@@ skipping 11 matching lines @@
 
 bool WebStateDelegateBridge::HandleContextMenu(
     WebState* source,
     const ContextMenuParams& params) {
   if ([delegate_ respondsToSelector:@selector(webState:handleContextMenu:)]) {
     return [delegate_ webState:source handleContextMenu:params];
   }
   return NO;
 }
 
+JavaScriptDialogPresenter* WebStateDelegateBridge::GetJavaScriptDialogPresenter(
+    WebState* source) {
+  if ([delegate_ respondsToSelector:
+        @selector(javaScriptDialogPresenterForWebState:)]) {
+    return [delegate_ javaScriptDialogPresenterForWebState:source];
+  }
+  return nullptr;
+}
+
 }  // web