Make VectorTraitsBase::IsDeletedValue accept a ref

This avoids passing an object by value (copy consturctor may not always exist).
No more need for overshadowing IsDeletedValue hack for VectorBackedLinkedListNode.

Change-Id: I5895a7e9b34f244e658bb319443b5c22451bd6ff
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2134750Reviewed-by: Keishi Hattori <keishi@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Yuki Shiino <yukishiino@chromium.org>
Commit-Queue: Kentaro Hara <haraken@chromium.org>
Auto-Submit: Bartek Nowierski <bartekn@chromium.org>
Cr-Commit-Position: refs/heads/master@{#756174}

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

@@ -79,16 +79,6 @@ struct VectorTraits<VectorBackedLinkedListNode<ValueType, Allocator>>
       VectorTraits<ValueType>::kCanCopyWithMemcpy;
   static const bool kCanMoveWithMemcpy =
       VectorTraits<ValueType>::kCanMoveWithMemcpy;
-
-  // Needs to be shadowing because |VectorTraitsBase::IsDeletedValue| uses call
-  // by value, which means we need to define copy constructor of
-  // |VectorBackedLinkedList|. We can remove this function if we change
-  // |VectorTraitsBase::IsDeletedValue| to use call by reference.
-  static bool IsDeletedValue(
-      const VectorBackedLinkedListNode<ValueType, Allocator>& node) {
-    NOTREACHED();
-    return false;
-  }
 };
 
 // VectorBackedLinkedList maintains a linked list through its contents such that

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

@@ -66,7 +66,7 @@ struct VectorTraitsBase {
 
   // Vectors do not support deleting values.
   static constexpr bool kCanHaveDeletedValue = false;
-  static bool IsDeletedValue(T value) { return false; }
+  static bool IsDeletedValue(const T& value) { return false; }
 };
 
 template <typename T>