[heap] Remove unused support for heap iterator size function.

src/globals.h

Index: src/globals.h
diff --git a/src/globals.h b/src/globals.h
index 486794df366e98aad5ea35c49a4eed83c1471b7f..41c33fa7ab1e301f16f902e023ddf778dcd9f762 100644
--- a/src/globals.h
+++ b/src/globals.h
@@ -572,11 +572,6 @@ struct CodeDesc {
 };
 
 
-// Callback function used for iterating objects in heap spaces,
-// for example, scanning heap objects.
-typedef int (*HeapObjectCallback)(HeapObject* obj);
-
-
 // Callback function used for checking constraints when copying/relocating
 // objects. Returns true if an object can be copied/relocated from its
 // old_addr to a new_addr.