Move the notion of current Tab from TabModel to WebStateList.

ios/shared/chrome/browser/tabs/web_state_list_order_controller.mm

 // 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.
 
 #import "ios/shared/chrome/browser/tabs/web_state_list_order_controller.h"
 
 #include <cstdint>
 
 #include "base/logging.h"
 #import "ios/shared/chrome/browser/tabs/web_state_list.h"
@@ skipping 22 matching lines @@
       opener, opener_index, true);
 
   int reference_index = list_child_index != WebStateList::kInvalidIndex
                             ? list_child_index
                             : opener_index;
 
   // Check for overflows (just a DCHECK as INT_MAX open WebState is unlikely).
   DCHECK_LT(reference_index, INT_MAX);
   return reference_index + 1;
 }
+
+int WebStateListOrderController::DetermineNewActiveIndex(
+    int removing_index) const {
+  DCHECK(web_state_list_->ContainsIndex(removing_index));
+  // First see if the index being removed has any "child" WebState. If it does,
+  // select the first WebState in that child group, not the next in the removed
+  // index group.
+  int index = web_state_list_->GetIndexOfNextWebStateOpenedBy(
+      web_state_list_->GetWebStateAt(removing_index), removing_index, false);
+
+  if (index != WebStateList::kInvalidIndex)
+    return GetValidIndex(index, removing_index);
+
+  web::WebState* opener =
+      web_state_list_->GetOpenerOfWebStateAt(removing_index);
+  if (opener) {
+    // If the WebState was in a group, shift selection to the next WebState in
+    // the group.
+    int index = web_state_list_->GetIndexOfNextWebStateOpenedBy(
+        opener, removing_index, false);
+
+    if (index != WebStateList::kInvalidIndex)
+      return GetValidIndex(index, removing_index);
+
+    // If there is no subsequent group member, just fall back to opener itself.
+    index = web_state_list_->GetIndexOfWebState(opener);
+    return GetValidIndex(index, removing_index);
+  }
+
+  // If this is the last WebState in the WebStateList, clear the selection.
+  if (web_state_list_->count() == 1)
+    return WebStateList::kInvalidIndex;
+
+  // No opener, fall through to the default handler, i.e. returning the previous
+  // WebState if the removed one is the last, otherwise returning the next one.
+  if (removing_index >= web_state_list_->count() - 1)
+    return removing_index - 1;
+
+  return removing_index;
+}
+
+int WebStateListOrderController::GetValidIndex(int index,
+                                               int removing_index) const {
+  if (removing_index < index)
+    return std::min(0, index - 1);
+  return index;
+}