Remove the dependency of Zone on Isolate

src/compiler/zone-pool.h

 // Copyright 2014 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef V8_COMPILER_ZONE_POOL_H_
 #define V8_COMPILER_ZONE_POOL_H_
 
 #include <map>
 #include <set>
 #include <vector>
@@ skipping 58 matching lines @@
 
  private:
   Zone* NewEmptyZone();
   void ReturnZone(Zone* zone);
 
   static const size_t kMaxUnusedSize = 3;
   typedef std::vector<Zone*> Unused;
   typedef std::vector<Zone*> Used;
   typedef std::vector<StatsScope*> Stats;
 
-  Isolate* const isolate_;
   Unused unused_;
   Used used_;
   Stats stats_;
   size_t max_allocated_bytes_;
   size_t total_deleted_bytes_;
 
   DISALLOW_COPY_AND_ASSIGN(ZonePool);
 };
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8
 
 #endif