[base] Move hashmap allocator to a field

src/zone.h

Index: src/zone.h
diff --git a/src/zone.h b/src/zone.h
index 78a2d52e273cc1761971644e47af044ea3b16f63..ee3592aaad49a34ee76ca94c2264499dd1be97c6 100644
--- a/src/zone.h
+++ b/src/zone.h
@@ -158,7 +158,7 @@ class ZoneScope final {
 
 // The ZoneAllocationPolicy is used to specialize generic data
 // structures to allocate themselves and their elements in the Zone.
-class ZoneAllocationPolicy final {
+class ZoneAllocationPolicy {
  public:
   explicit ZoneAllocationPolicy(Zone* zone) : zone_(zone) { }
   void* New(size_t size) { return zone()->New(size); }