Remove the dependency of Zone on Isolate

src/compiler/zone-pool.h

Index: src/compiler/zone-pool.h
diff --git a/src/compiler/zone-pool.h b/src/compiler/zone-pool.h
index 8b43265c1a8ae24605de403c82642243568ecad7..d2fbbbd92f716e1f438a63c64f1f8d0750a61dad 100644
--- a/src/compiler/zone-pool.h
+++ b/src/compiler/zone-pool.h
@@ -60,7 +60,7 @@ class ZonePool FINAL {
     DISALLOW_COPY_AND_ASSIGN(StatsScope);
   };
 
-  explicit ZonePool(Isolate* isolate);
+  ZonePool();
   ~ZonePool();
 
   size_t GetMaxAllocatedBytes();
@@ -76,7 +76,6 @@ class ZonePool FINAL {
   typedef std::vector<Zone*> Used;
   typedef std::vector<StatsScope*> Stats;
 
-  Isolate* const isolate_;
   Unused unused_;
   Used used_;
   Stats stats_;