Add public API for handling Javascript alerts and prompt.

ios/web/web_state/web_state_delegate_bridge_unittest.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 <memory>
 
 #import "base/mac/scoped_nsobject.h"
 #include "base/strings/utf_string_conversions.h"
@@ skipping 50 matching lines @@
   EXPECT_EQ(context_menu_params.menu_title, result_params->menu_title);
   EXPECT_EQ(context_menu_params.link_url, result_params->link_url);
   EXPECT_EQ(context_menu_params.src_url, result_params->src_url);
   EXPECT_EQ(context_menu_params.referrer_policy,
             result_params->referrer_policy);
   EXPECT_EQ(context_menu_params.view, result_params->view);
   EXPECT_EQ(context_menu_params.location.x, result_params->location.x);
   EXPECT_EQ(context_menu_params.location.y, result_params->location.y);
 }
 
+// Tests |GetJavaScriptDialogPresenter| forwarding.
+TEST_F(WebStateDelegateBridgeTest, GetJavaScriptDialogPresenter) {
+  EXPECT_FALSE([delegate_ javaScriptDialogPresenterRequested]);
+  bridge_->GetJavaScriptDialogPresenter(nullptr);
+  EXPECT_TRUE([delegate_ javaScriptDialogPresenterRequested]);
+}
+
 }  // namespace web