Move the notion of current Tab from TabModel to WebStateList.

ios/chrome/browser/tabs/tab_model_order_controller.mm

-// Copyright 2012 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/tabs/tab_model_order_controller.h"
-
-#include "base/logging.h"
-
-#if !defined(__has_feature) || !__has_feature(objc_arc)
-#error "This file requires ARC support."
-#endif
-
-@implementation TabModelOrderController {
-  __weak TabModel* model_;
-}
-
-- (instancetype)initWithTabModel:(TabModel*)model {
-  DCHECK(model);
-  if ((self = [super init]))
-    model_ = model;
-  return self;
-}
-
-- (Tab*)determineNewSelectedTabFromRemovedTab:(Tab*)removedTab {
-  // While the desktop version of this code deals in indices, this deals in
-  // actual tabs. As a result, the tab only needs to change iff the selection
-  // is the tab that's removed.
-  if (removedTab != model_.currentTab)
-    return model_.currentTab;
-
-  const NSUInteger numberOfTabs = model_.count;
-  if (numberOfTabs < 2)
-    return nil;
-
-  DCHECK(numberOfTabs >= 2);
-  DCHECK(removedTab == model_.currentTab);
-
-  // First see if the tab being removed has any "child" tabs. If it does, we
-  // want to select the first in that child group, not the next tab in the same
-  // group of the removed tab.
-  Tab* firstChild = [model_ nextTabWithOpener:removedTab afterTab:nil];
-  if (firstChild)
-    return firstChild;
-  Tab* parentTab = [model_ openerOfTab:removedTab];
-  if (parentTab) {
-    // If the tab was in a group, shift selection to the next tab in the group.
-    Tab* nextTab = [model_ nextTabWithOpener:parentTab afterTab:removedTab];
-    if (nextTab)
-      return nextTab;
-
-    // If we can't find a subsequent group member, just fall back to the
-    // parentTab itself. Note that we use "group" here since opener is
-    // reset by select operations.
-    return parentTab;
-  }
-
-  // No opener set, fall through to the default handler...
-  NSUInteger selectedIndex = [model_ indexOfTab:removedTab];
-  DCHECK(selectedIndex <= numberOfTabs - 1);
-  // Is the closing tab the last one? If so, return the penultimate tab.
-  if (selectedIndex == numberOfTabs - 1)
-    return [model_ tabAtIndex:selectedIndex - 1];
-  // Otherwise return the next tab after the current tab.
-  return [model_ tabAtIndex:selectedIndex + 1];
-}
-
-@end