Get rid of ZoneScope completely.

src/hydrogen-gvn.h

Index: src/hydrogen-gvn.h
diff --git a/src/hydrogen-gvn.h b/src/hydrogen-gvn.h
index 2d6075e7157a8d558a8d9f496e1e7c6b9e648c8d..a0db42760352bacccc431ee5507d32c272d77e05 100644
--- a/src/hydrogen-gvn.h
+++ b/src/hydrogen-gvn.h
@@ -100,14 +100,13 @@ class HGlobalValueNumberer BASE_EMBEDDED {
 
   HGraph* graph() { return graph_; }
   CompilationInfo* info() { return info_; }
-  Zone* phase_zone() const { return info_->phase_zone(); }
+  Zone* zone() { return &zone_; }
 
   HGraph* graph_;
   CompilationInfo* info_;
   bool removed_side_effects_;
 
-  Zone* phase_zone_;
-  ZoneScope phase_zone_scope_;
+  Zone zone_;
 
   // A map of block IDs to their side effects.
   ZoneList<GVNFlagSet> block_side_effects_;