[ios] Implemented -[ShellEarlGrey loadURL:].

ios/web/shell/test/app/navigation_test_util.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.
 
 #include "ios/web/shell/test/app/navigation_test_util.h"
 
 #import "ios/web/public/test/navigation_test_util.h"
 #import "ios/web/shell/test/app/web_shell_test_util.h"
 
 namespace web {
 namespace shell_test_util {
 
 void LoadUrl(const GURL& url) {
   web::test::LoadUrl(GetCurrentWebState(), url);
 }
 
+bool IsLoading() {
+  return GetCurrentWebState()->IsLoading();
+}
+
 }  // namespace shell_test_util
 }  // namespace web