Upstream Chrome on iOS source code [10/11].

ios/chrome/browser/ui/util/top_view_controller.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/chrome/browser/ui/util/top_view_controller.h"
+
+namespace top_view_controller {
+
+UIViewController* TopPresentedViewController() {
+  return TopPresentedViewControllerFrom(
+      [[UIApplication sharedApplication] keyWindow].rootViewController);
+}
+UIViewController* TopPresentedViewControllerFrom(
+    UIViewController* base_view_controller) {
+  UIViewController* topController = base_view_controller;
+  for (UIViewController* controller = [topController presentedViewController];
+       controller && ![controller isBeingDismissed];
+       controller = [controller presentedViewController]) {
+    topController = controller;
+  }
+  return topController;
+}
+
+}  // namespace top_view_controller