Use base::flat_map for base::Value dictionary storage.

base/values.cc

Index: base/values.cc
diff --git a/base/values.cc b/base/values.cc
index d492a474631c841442d72851ebff94ff3d163241..23d2b384639c6dbd7e7bb27e75bf67330a573b82 100644
--- a/base/values.cc
+++ b/base/values.cc
@@ -167,6 +167,10 @@ Value::Value(std::vector<char>&& in_blob) noexcept : type_(Type::BINARY) {
   binary_value_.Init(std::move(in_blob));
 }
 
+Value::Value(DictStorage&& in_dict) noexcept : type_(Type::DICTIONARY) {
+  dict_ptr_.Init(MakeUnique<DictStorage>(std::move(in_dict)));
+}
+
 Value& Value::operator=(const Value& that) {
   if (type_ == that.type_) {
     InternalCopyAssignFromSameType(that);