Make Zone::allocation_size work with parallel zones.

src/zone.h

Index: src/zone.h
diff --git a/src/zone.h b/src/zone.h
index 01e887e779d70377db2c8a80e653e94b53d7f2e9..6d94754e4b47b3212dee5ec215cf186d9e2bcef6 100644
--- a/src/zone.h
+++ b/src/zone.h
@@ -86,9 +86,9 @@ class Zone {
 
   inline void adjust_segment_bytes_allocated(int delta);
 
-  inline Isolate* isolate() { return isolate_; }
+  inline unsigned allocation_size() { return allocation_size_; }
 
-  static unsigned allocation_size_;
+  inline Isolate* isolate() { return isolate_; }
 
  private:
   friend class Isolate;
@@ -111,6 +111,9 @@ class Zone {
   // Report zone excess when allocation exceeds this limit.
   int zone_excess_limit_;
 
+  // The number of bytes allocated in this zone so far.
+  unsigned allocation_size_;
+
   // The number of bytes allocated in segments.  Note that this number
   // includes memory allocated from the OS but not yet allocated from
   // the zone.