VM: Make ZoneAllocated-objects that don't need a vtable smaller.

runtime/vm/allocation.h

Index: runtime/vm/allocation.h
===================================================================
--- runtime/vm/allocation.h	(revision 36988)
+++ runtime/vm/allocation.h	(working copy)
@@ -89,9 +89,6 @@
 class ZoneAllocated {
  public:
   ZoneAllocated() { }
-  // It would be ideal if the destructor method could be made private,
-  // but the g++ compiler complains when this is subclassed.
-  virtual ~ZoneAllocated();
 
   // Implicitly allocate the object in the current zone.
   void* operator new(uword size);