[ios Mojo] Enabled Mojo WebUI by default.

ios/web/webui/web_ui_mojo_inttest.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 <memory>
 
 #include "base/run_loop.h"
 #import "base/test/ios/wait_util.h"
 #include "ios/public/provider/web/web_ui_ios_controller.h"
 #include "ios/public/provider/web/web_ui_ios_controller_factory.h"
@@ skipping 130 matching lines @@
  private:
   std::unique_ptr<WebStateImpl> web_state_;
   std::unique_ptr<TestUIHandler> ui_handler_;
 };
 
 // Tests that JS can send messages to the native code and vice versa.
 // TestUIHandler is used for communication and test suceeds only when
 // |TestUIHandler| sucessfully receives "ack" message from WebUI page.
 TEST_F(WebUIMojoTest, MessageExchange) {
   web_state()->SetWebUsageEnabled(true);
-  web_state()->GetWebController().useMojoForWebUI = YES;
   web_state()->GetView();  // WebState won't load URL without view.
   NavigationManager::WebLoadParams load_params(GURL(
       url::SchemeHostPort(kTestWebUIScheme, kTestWebUIURLHost, 0).Serialize()));
   web_state()->GetNavigationManager()->LoadURLWithParams(load_params);
 
   // Wait until |TestUIHandler| receives "ack" message from WebUI page.
   base::test::ios::WaitUntilCondition(^{
     base::RunLoop().RunUntilIdle();
     return test_ui_handler()->IsFinReceived();
   });
 }
 
 }  // namespace web