Replaced different means of zone pooling/reusing by one zone segment pool

src/isolate.h

Index: src/isolate.h
diff --git a/src/isolate.h b/src/isolate.h
index 169f91fabe438377220fe8c9094931b1d318aa5d..96bf37b7d47caf12fc1ed4b70117635846401fd0 100644
--- a/src/isolate.h
+++ b/src/isolate.h
@@ -893,7 +893,6 @@ class Isolate {
     DCHECK(handle_scope_implementer_);
     return handle_scope_implementer_;
   }
-  Zone* runtime_zone() { return runtime_zone_; }
 
   UnicodeCache* unicode_cache() {
     return unicode_cache_;
@@ -1334,7 +1333,6 @@ class Isolate {
   HandleScopeImplementer* handle_scope_implementer_;
   UnicodeCache* unicode_cache_;
   AccountingAllocator* allocator_;
-  Zone* runtime_zone_;
   InnerPointerToCodeCache* inner_pointer_to_code_cache_;
   GlobalHandles* global_handles_;
   EternalHandles* eternal_handles_;