Remove the dependency of Zone on Isolate

src/zone.h

Index: src/zone.h
diff --git a/src/zone.h b/src/zone.h
index 6f552b6524413c08172d25e6fd8c70ca2b27f85a..2a730f9ee078c478a912cbe584bcf2c263828af5 100644
--- a/src/zone.h
+++ b/src/zone.h
@@ -36,7 +36,7 @@ class Isolate;
 
 class Zone {
  public:
-  explicit Zone(Isolate* isolate);
+  Zone();
   ~Zone();
   // Allocate 'size' bytes of memory in the Zone; expands the Zone by
   // allocating new segments of memory on demand using malloc().
@@ -65,8 +65,6 @@ class Zone {
 
   inline unsigned allocation_size() const { return allocation_size_; }
 
-  inline Isolate* isolate() const { return isolate_; }
-
  private:
   friend class Isolate;
 
@@ -120,7 +118,6 @@ class Zone {
   Address limit_;
 
   Segment* segment_head_;
-  Isolate* isolate_;
 };