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

base/json/json_parser.cc

Index: base/json/json_parser.cc
diff --git a/base/json/json_parser.cc b/base/json/json_parser.cc
index a91038cee986b4bb02a4a1d9c41923c9a92998a2..be7d21dbb343d7a7afd1f2f24651073e1702ef61 100644
--- a/base/json/json_parser.cc
+++ b/base/json/json_parser.cc
@@ -344,7 +344,7 @@ std::unique_ptr<Value> JSONParser::ConsumeDictionary() {
     return nullptr;
   }
 
-  std::unique_ptr<DictionaryValue> dict(new DictionaryValue);
+  std::vector<Value::DictStorage::value_type> dict_storage;
 
   NextChar();
   Token token = GetNextToken();
@@ -376,7 +376,7 @@ std::unique_ptr<Value> JSONParser::ConsumeDictionary() {
       return nullptr;
     }
 
-    dict->SetWithoutPathExpansion(key.AsStringPiece(), std::move(value));
+    dict_storage.emplace_back(key.DestructiveAsString(), std::move(value));
 
     NextChar();
     token = GetNextToken();
@@ -393,7 +393,8 @@ std::unique_ptr<Value> JSONParser::ConsumeDictionary() {
     }
   }
 
-  return std::move(dict);
+  return MakeUnique<Value>(
+      Value::DictStorage(std::move(dict_storage), KEEP_LAST_OF_DUPES));
 }
 
 std::unique_ptr<Value> JSONParser::ConsumeList() {