[ObjC ARC] Converts ios/chrome/test:test_support to ARC.

ios/chrome/test/scoped_key_window.h

 // Copyright 2017 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.
 
 #ifndef IOS_CHROME_TEST_SCOPED_KEY_WINDOW_H_
 #define IOS_CHROME_TEST_SCOPED_KEY_WINDOW_H_
 
 #import <UIKit/UIKit.h>
 
 // Sets temporary key window and returns it via Get method. Saves the current
 // key window and restores it on destruction.
 class ScopedKeyWindow {
  public:
   explicit ScopedKeyWindow()
       : current_key_window_(
             [[UIWindow alloc] initWithFrame:[UIScreen mainScreen].bounds]),
         original_key_window_([UIApplication sharedApplication].keyWindow) {
     [current_key_window_ makeKeyAndVisible];
   }
   ~ScopedKeyWindow() { [original_key_window_ makeKeyAndVisible]; }
   UIWindow* Get() { return current_key_window_; }
 
  private:
-  UIWindow* current_key_window_;
-  UIWindow* original_key_window_;
+  __strong UIWindow* current_key_window_;
+  __strong UIWindow* original_key_window_;
 };
 
 #endif  // IOS_CHROME_TEST_SCOPED_KEY_WINDOW_H_