Move the notion of current Tab from TabModel to WebStateList.

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

Index: ios/shared/chrome/browser/tabs/web_state_list_order_controller.mm
diff --git a/ios/shared/chrome/browser/tabs/web_state_list_order_controller.mm b/ios/shared/chrome/browser/tabs/web_state_list_order_controller.mm
index cafb8a4a86b9f1fd5e5d38f0df681f429a54a3aa..479224619fed6401e87a2324b73f5f604660b639 100644
--- a/ios/shared/chrome/browser/tabs/web_state_list_order_controller.mm
+++ b/ios/shared/chrome/browser/tabs/web_state_list_order_controller.mm
@@ -40,3 +40,50 @@ int WebStateListOrderController::DetermineInsertionIndex(
   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;
+}