Added a function |clear| to VectorBackedLinkedList

Implemented a functoin |clear|, which removes all elements in a linked list.

Change-Id: I94fc761eb7f779a532ccd7846254aff4f703c909
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2094897
Commit-Queue: Kei Nakashima <keinakashima@google.com>
Reviewed-by: Yuki Shiino <yukishiino@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#748525}

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

@@ -140,6 +140,15 @@ class VectorBackedLinkedList {
     erase(--cend());
   }
 
+  // Removes all elements in a linked list.
+  void clear() {
+    nodes_.clear();
+    // Reinserts anchor so that we can insert elements after this operation.
+    nodes_.push_back(Node(anchor_index_, anchor_index_));
+    free_head_index_ = anchor_index_;
+    size_ = 0;
+  }
+
  private:
   bool IsFreeListEmpty() const { return free_head_index_ == anchor_index_; }
 

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

@@ -193,6 +193,27 @@ TEST(VectorBackedLinkedList, PopBack) {
   EXPECT_TRUE(list.empty());
 }
 
+TEST(VectorBackedLinkedList, Clear) {
+  using List = VectorBackedLinkedList<int>;
+  List list;
+
+  list.push_back(1);
+  list.push_back(2);
+  list.push_back(3);
+
+  EXPECT_EQ(list.size(), 3u);
+
+  list.clear();
+  EXPECT_EQ(list.size(), 0u);
+  EXPECT_TRUE(list.empty());
+
+  EXPECT_TRUE(list.begin() == list.end());
+  list.push_back(1);
+  EXPECT_EQ(list.front(), 1);
+  EXPECT_EQ(list.back(), 1);
+  EXPECT_EQ(list.size(), 1u);
+}
+
 TEST(VectorBackedLinkedList, Iterator) {
   using List = VectorBackedLinkedList<int>;
   List list;