Fixed a bug in |VectorBackedLinkedList::insert|

In the current code, |insert| doesn't work correctly since it does not update prev/next indices when free list is not empty.
This CL fixes it.

Change-Id: I6355bfd61e4a7b8f928a462e45fb98702f005f03
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2094491Reviewed-by: Yuki Shiino <yukishiino@chromium.org>
Reviewed-by: Bartek Nowierski <bartekn@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Kei Nakashima <keinakashima@google.com>
Cr-Commit-Position: refs/heads/master@{#749007}

third_party/blink/renderer/platform/wtf/vector_backed_linked_list.h

@@ -408,14 +408,10 @@ typename VectorBackedLinkedList<T>::iterator VectorBackedLinkedList<T>::insert(
     IncomingValueType&& value) {
   wtf_size_t position_index = position.GetIndex();
   wtf_size_t prev_index = nodes_[position_index].prev_index_;
-  Node& prev = nodes_[prev_index];
-  Node& next = nodes_[position_index];
 
   wtf_size_t new_entry_index;
   if (IsFreeListEmpty()) {
     new_entry_index = nodes_.size();
-    prev.next_index_ = new_entry_index;
-    next.prev_index_ = new_entry_index;
     nodes_.push_back(Node(prev_index, position_index,
                           std::forward<IncomingValueType>(value)));
   } else {
@@ -425,6 +421,8 @@ typename VectorBackedLinkedList<T>::iterator VectorBackedLinkedList<T>::insert(
     free_head = Node(prev_index, position_index,
                      std::forward<IncomingValueType>(value));
   }
+  nodes_[prev_index].next_index_ = new_entry_index;
+  nodes_[position_index].prev_index_ = new_entry_index;
   size_++;
   return iterator(new_entry_index, this);
 }

third_party/blink/renderer/platform/wtf/vector_backed_linked_list_test.cc

@@ -143,6 +143,12 @@ TEST(VectorBackedLinkedList, Erase) {
   list.push_back(5);
   EXPECT_EQ(list.size(), 5u);
 
+  int i = 1;
+  for (auto element : list) {
+    EXPECT_EQ(element, i);
+    i++;
+  }
+
   List::iterator target = list.begin();
   ++target;
   it = list.erase(target);  // list = {1, 3, 4, 5}
@@ -162,6 +168,10 @@ TEST(VectorBackedLinkedList, Erase) {
   EXPECT_EQ(*it, 5);
   ++it;
   EXPECT_TRUE(it == list.end());
+
+  list.push_back(6);
+  EXPECT_EQ(list.front(), 3);
+  EXPECT_EQ(list.back(), 6);
 }
 
 TEST(VectorBackedLinkedList, PopFront) {