Add a safe API for creating ASCII-only strings

src/factory.h

Index: src/factory.h
diff --git a/src/factory.h b/src/factory.h
index dd613b71e49a50e8532293afb32a095fe1b98fc1..2cd092a56f47f03405b7d48634ad1fb91f9e649d 100644
--- a/src/factory.h
+++ b/src/factory.h
@@ -117,6 +117,10 @@ class Factory {
       Vector<const char> str,
       PretenureFlag pretenure = NOT_TENURED);
 
+  Handle<String> NewStringFromAsciiSafe(
+      Vector<const char> str,
+      PretenureFlag pretenure = NOT_TENURED);
+
   // UTF8 strings are pretenured when used for regexp literal patterns and
   // flags in the parser.
   Handle<String> NewStringFromUtf8(