[ios] Restore the original key window at the end of the AlertCoordinator tests.

ios/chrome/browser/ui/alert_coordinator/alert_coordinator_unittest.mm

Index: ios/chrome/browser/ui/alert_coordinator/alert_coordinator_unittest.mm
diff --git a/ios/chrome/browser/ui/alert_coordinator/alert_coordinator_unittest.mm b/ios/chrome/browser/ui/alert_coordinator/alert_coordinator_unittest.mm
index ba8ad718d6db0397b0f208b5d5917c4317a0f85e..9a984b9f2106d899d2d371d1d0f90225d539a5fd 100644
--- a/ios/chrome/browser/ui/alert_coordinator/alert_coordinator_unittest.mm
+++ b/ios/chrome/browser/ui/alert_coordinator/alert_coordinator_unittest.mm
@@ -23,12 +23,16 @@
 class AlertCoordinatorTest : public PlatformTest {
  protected:
   AlertCoordinatorTest() {
+    // Save the current key window and restore it after the test.
+    previous_key_window_ = [[UIApplication sharedApplication] keyWindow];
     window_ = [[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]];
     [window_ makeKeyAndVisible];
     view_controller_ = [[UIViewController alloc] init];
     [window_ setRootViewController:view_controller_];
   }
 
+  ~AlertCoordinatorTest() override { [previous_key_window_ makeKeyAndVisible]; }
+
   void startAlertCoordinator() { [alert_coordinator_ start]; }
 
   UIViewController* getViewController() { return view_controller_; }
@@ -48,6 +52,7 @@ class AlertCoordinatorTest : public PlatformTest {
   }
 
  private:
+  UIWindow* previous_key_window_;
   AlertCoordinator* alert_coordinator_;
   UIWindow* window_;
   UIViewController* view_controller_;