Handlify concat string and substring.

src/heap.h

Index: src/heap.h
diff --git a/src/heap.h b/src/heap.h
index 96cda586b7c9ea464bc7b1fd39d38c160b34e207..3612eabf39c6e5d5840c3ef95ad52634240d4445 100644
--- a/src/heap.h
+++ b/src/heap.h
@@ -1073,25 +1073,6 @@ 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