Upstream Chrome on iOS source code [5/11].

ios/chrome/browser/ui/main/main_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/main/main_view_controller.h"
+
+#import "base/logging.h"
+
+@implementation MainViewController
+
+- (UIViewController*)activeViewController {
+  return [self.childViewControllers firstObject];
+}
+
+- (void)setActiveViewController:(UIViewController*)activeViewController {
+  DCHECK(activeViewController);
+  if (self.activeViewController == activeViewController)
+    return;
+
+  // TODO(crbug.com/546189): DCHECK here that there isn't a modal view
+  // controller showing once the known violations of that are fixed.
+
+  // Remove the current active view controller, if any.
+  if (self.activeViewController) {
+    [self.activeViewController willMoveToParentViewController:nil];
+    [self.activeViewController.view removeFromSuperview];
+    [self.activeViewController removeFromParentViewController];
+  }
+
+  DCHECK(self.activeViewController == nil);
+  DCHECK(self.view.subviews.count == 0);
+
+  // Add the new active view controller.
+  [self addChildViewController:activeViewController];
+  self.activeViewController.view.frame = self.view.bounds;
+  [self.view addSubview:self.activeViewController.view];
+  [activeViewController didMoveToParentViewController:self];
+
+  // Let the system know that the child has changed so appearance updates can
+  // be made.
+  [self setNeedsStatusBarAppearanceUpdate];
+
+  DCHECK(self.activeViewController == activeViewController);
+}
+
+#pragma mark - UIViewController methods
+
+- (void)presentViewController:(UIViewController*)viewControllerToPresent
+                     animated:(BOOL)flag
+                   completion:(void (^)())completion {
+  [self.activeViewController presentViewController:viewControllerToPresent
+                                          animated:flag
+                                        completion:completion];
+}
+
+- (void)dismissViewControllerAnimated:(BOOL)flag
+                           completion:(void (^)())completion {
+  [self.activeViewController dismissViewControllerAnimated:flag
+                                                completion:completion];
+}
+
+- (UIViewController*)childViewControllerForStatusBarHidden {
+  return self.activeViewController;
+}
+
+- (UIViewController*)childViewControllerForStatusBarStyle {
+  return self.activeViewController;
+}
+
+- (BOOL)shouldAutorotate {
+  return self.activeViewController
+             ? [self.activeViewController shouldAutorotate]
+             : [super shouldAutorotate];
+}
+
+@end