Move function length tracking from Scope to (Pre)?ParserFormalParameters.

src/parsing/parser.h

Index: src/parsing/parser.h
diff --git a/src/parsing/parser.h b/src/parsing/parser.h
index 9cda51672bd767a72d2a8ed31c2367a4488cb550..cae6ec03f476ea9e5f8948eb969bbebaa4deebbf 100644
--- a/src/parsing/parser.h
+++ b/src/parsing/parser.h
@@ -134,7 +134,6 @@ struct ParserFormalParameters : FormalParametersBase {
       : FormalParametersBase(scope), params(4, scope->zone()) {}
   ZoneList<Parameter> params;
 
-  int Arity() const { return params.length(); }
   const Parameter& at(int i) const { return params[i]; }
 };
 
@@ -994,6 +993,7 @@ class Parser : public ParserBase<Parser> {
                                     Expression* initializer,
                                     int initializer_end_position,
                                     bool is_rest) {
+    parameters->UpdateArityAndFunctionLength(initializer != nullptr, is_rest);
     bool is_simple = pattern->IsVariableProxy() && initializer == nullptr;
     const AstRawString* name = is_simple
                                    ? pattern->AsVariableProxy()->raw_name()