Add public API for handling Javascript alerts and prompt.

ios/web/web_state/web_state_delegate_stub.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/web_state/web_state_delegate_stub.h"
 
 #import "ios/web/public/web_state/web_state.h"
 #import "ios/web/public/web_state/context_menu_params.h"
 
 @implementation CRWWebStateDelegateStub {
   // Backs up the property with the same name.
   std::unique_ptr<web::ContextMenuParams> _contextMenuParams;
+  // Backs up the property with the same name.
+  BOOL _javaScriptDialogPresenterRequested;
 }
 
 @synthesize webState = _webState;
 @synthesize changedProgress = _changedProgress;
 
 - (void)webState:(web::WebState*)webState didChangeProgress:(double)progress {
   _webState = webState;
   _changedProgress = progress;
 }
 
 - (BOOL)webState:(web::WebState*)webState
     handleContextMenu:(const web::ContextMenuParams&)params {
   _webState = webState;
   _contextMenuParams.reset(new web::ContextMenuParams(params));
   return YES;
 }
 
+- (web::JavaScriptDialogPresenter*)javaScriptDialogPresenterForWebState:
+    (web::WebState*)webState {
+  _webState = webState;
+  _javaScriptDialogPresenterRequested = YES;
+  return nil;
+}
+
 - (web::ContextMenuParams*)contextMenuParams {
   return _contextMenuParams.get();
 }
 
+- (BOOL)javaScriptDialogPresenterRequested {
+  return _javaScriptDialogPresenterRequested;
+}
+
 @end