Add a safe API for creating ASCII-only strings

src/heap.h

Index: src/heap.h
diff --git a/src/heap.h b/src/heap.h
index 606f78731ae7396d3f7d168c64c16ad0e7c4556d..d244a6b2fd683e3976450c15b51e65ebcf7cd148 100644
--- a/src/heap.h
+++ b/src/heap.h
@@ -701,6 +701,9 @@ class Heap {
   MUST_USE_RESULT MaybeObject* AllocateStringFromOneByte(
       Vector<const char> str,
       PretenureFlag pretenure = NOT_TENURED);
+  MUST_USE_RESULT MaybeObject* AllocateStringFromAsciiSafe(
+      Vector<const char> str,
+      PretenureFlag pretenure = NOT_TENURED);
   MUST_USE_RESULT inline MaybeObject* AllocateStringFromUtf8(
       Vector<const char> str,
       PretenureFlag pretenure = NOT_TENURED);