Replace boolean indications of strict mode by an enum value.

src/ast.h

Index: src/ast.h
diff --git a/src/ast.h b/src/ast.h
index 2ae1364d32d98df66b93b0b0fd81e71a0099e151..fda28011b2f343eea7be0b0ee86828c0eea435d1 100644
--- a/src/ast.h
+++ b/src/ast.h
@@ -1652,7 +1652,8 @@ class FunctionLiteral: public Expression {
   int end_position() const { return end_position_; }
   bool is_expression() const { return is_expression_; }
   bool is_anonymous() const { return is_anonymous_; }
-  bool strict_mode() const;
+  bool strict_mode() const { return strict_mode_flag() == kStrictMode; }
+  StrictModeFlag strict_mode_flag() const;
 
   int materialized_literal_count() { return materialized_literal_count_; }
   int expected_property_count() { return expected_property_count_; }