Reland of Change base::Value::ListStorage to std::vector<base::Value>

base/values.h

Index: base/values.h
diff --git a/base/values.h b/base/values.h
index ace8b43651a81677233d07a10a2e44f7a07b930b..075bc9fbb8fed7cd14095f1de4daa8a3ab9c13ea 100644
--- a/base/values.h
+++ b/base/values.h
@@ -49,7 +49,7 @@ class Value;
 class BASE_EXPORT Value {
  public:
   using DictStorage = base::flat_map<std::string, std::unique_ptr<Value>>;
-  using ListStorage = std::vector<std::unique_ptr<Value>>;
+  using ListStorage = std::vector<Value>;
 
   enum class Type {
     NONE = 0,
@@ -390,9 +390,15 @@ class BASE_EXPORT ListValue : public Value {
   // Returns the number of Values in this list.
   size_t GetSize() const { return list_->size(); }
 
+  // Returns the capacity of storage for Values in this list.
+  size_t capacity() const { return list_->capacity(); }
+
   // Returns whether the list is empty.
   bool empty() const { return list_->empty(); }
 
+  // Reserves storage for at least |n| values.
+  void Reserve(size_t n);
+
   // Sets the list item at the given index to be the Value specified by
   // the value given.  If the index beyond the current end of the list, null
   // Values will be used to pad out the list.