Always allocate symbols in old space.

src/heap.h

Index: src/heap.h
diff --git a/src/heap.h b/src/heap.h
index 7a1d3c577ebeed2b3418001cb91fea4d07ff8fef..8992e318e274e5b79be10a3480f9ad0f528205af 100644
--- a/src/heap.h
+++ b/src/heap.h
@@ -882,12 +882,11 @@ class Heap {
       void* external_pointer,
       PretenureFlag pretenure);
 
-  // Allocate a symbol.
+  // Allocate a symbol in old space.
   // Returns Failure::RetryAfterGC(requested_bytes, space) if the allocation
   // failed.
   // Please note this does not perform a garbage collection.
-  MUST_USE_RESULT MaybeObject* AllocateSymbol(
-      PretenureFlag pretenure = NOT_TENURED);
+  MUST_USE_RESULT MaybeObject* AllocateSymbol();
 
   // Allocate a tenured JS global property cell.
   // Returns Failure::RetryAfterGC(requested_bytes, space) if the allocation