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

src/compiler/pipeline-statistics.h

Index: src/compiler/pipeline-statistics.h
diff --git a/src/compiler/pipeline-statistics.h b/src/compiler/pipeline-statistics.h
index b09e2363d66f4d8feb53ebda2feb2b1b55f41c9f..a9931ebed75f28fc282c1fb82c36aded6e0504a9 100644
--- a/src/compiler/pipeline-statistics.h
+++ b/src/compiler/pipeline-statistics.h
@@ -10,7 +10,7 @@
 
 #include "src/base/platform/elapsed-timer.h"
 #include "src/compilation-statistics.h"
-#include "src/compiler/zone-stats.h"
+#include "src/compiler/zone-pool.h"
 
 namespace v8 {
 namespace internal {
@@ -20,7 +20,7 @@
 
 class PipelineStatistics : public Malloced {
  public:
-  PipelineStatistics(CompilationInfo* info, ZoneStats* zone_stats);
+  PipelineStatistics(CompilationInfo* info, ZonePool* zone_pool);
   ~PipelineStatistics();
 
   void BeginPhaseKind(const char* phase_kind_name);
@@ -39,7 +39,7 @@
     void End(PipelineStatistics* pipeline_stats,
              CompilationStatistics::BasicStats* diff);
 
-    std::unique_ptr<ZoneStats::StatsScope> scope_;
+    std::unique_ptr<ZonePool::StatsScope> scope_;
     base::ElapsedTimer timer_;
     size_t outer_zone_initial_size_;
     size_t allocated_bytes_at_start_;
@@ -57,7 +57,7 @@
 
   Isolate* isolate_;
   Zone* outer_zone_;
-  ZoneStats* zone_stats_;
+  ZonePool* zone_pool_;
   CompilationStatistics* compilation_stats_;
   std::string function_name_;