Simplify scheduler API by removing zone scopes.

src/compiler/scheduler.h

Index: src/compiler/scheduler.h
diff --git a/src/compiler/scheduler.h b/src/compiler/scheduler.h
index 2839a0cb7a0585eba86bd338bbed5ec7f5e0bc8a..e75469090f75ae3736f1adf47cbf9b547bd3d8b2 100644
--- a/src/compiler/scheduler.h
+++ b/src/compiler/scheduler.h
@@ -24,11 +24,10 @@ class Scheduler {
  public:
   // The complete scheduling algorithm. Creates a new schedule and places all
   // nodes from the graph into it.
-  static Schedule* ComputeSchedule(ZonePool* zone_pool, Graph* graph);
+  static Schedule* ComputeSchedule(Zone* zone, Graph* graph);
 
   // Compute the RPO of blocks in an existing schedule.
-  static BasicBlockVector* ComputeSpecialRPO(ZonePool* zone_pool,
-                                             Schedule* schedule);
+  static BasicBlockVector* ComputeSpecialRPO(Zone* zone, Schedule* schedule);
 
  private:
   // Placement of a node changes during scheduling. The placement state