Reland "Handlify concat string and substring."

src/heap.h

Index: src/heap.h
diff --git a/src/heap.h b/src/heap.h
index d2d69618934f3eb7799423a1ecec2941ce750bd3..e55b3db83d6fa4aa70295d8dd453fa39f62aec21 100644
--- a/src/heap.h
+++ b/src/heap.h
@@ -1071,25 +1071,6 @@ class Heap {
       Object* stack_trace,
       Object* stack_frames);
 
-  // Allocates a new cons string object.
-  // Returns Failure::RetryAfterGC(requested_bytes, space) if the allocation
-  // failed.
-  // Please note this does not perform a garbage collection.
-  MUST_USE_RESULT MaybeObject* AllocateConsString(String* first,
-                                                  String* second);
-
-  // Allocates a new sub string object which is a substring of an underlying
-  // string buffer stretching from the index start (inclusive) to the index
-  // end (exclusive).
-  // Returns Failure::RetryAfterGC(requested_bytes, space) if the allocation
-  // failed.
-  // Please note this does not perform a garbage collection.
-  MUST_USE_RESULT MaybeObject* AllocateSubString(
-      String* buffer,
-      int start,
-      int end,
-      PretenureFlag pretenure = NOT_TENURED);
-
   // Allocate a new external string object, which is backed by a string
   // resource that resides outside the V8 heap.
   // Returns Failure::RetryAfterGC(requested_bytes, space) if the allocation