Fix memory leaks in ~Zone and ~Isolate

src/zone.h

Index: src/zone.h
diff --git a/src/zone.h b/src/zone.h
index abb53ad46d3b236096b13155076d3b53894d1f31..faad3b7ccd04a056663a69b4a548c3d8a4d9b449 100644
--- a/src/zone.h
+++ b/src/zone.h
@@ -65,9 +65,13 @@ class Zone {
   template <typename T>
   inline T* NewArray(int length);
 
-  // Delete all objects and free all memory allocated in the Zone.
+  // Deletes all objects and free all memory allocated in the Zone. Keeps one
+  // small (size <= kMaximumKeptSegmentSize) segment around if it finds one.
   void DeleteAll();
 
+  // Deletes the last small segment kept around by DeleteAll().
+  void DeleteKeptSegment();
+
   // Returns true if more memory has been allocated in zones than
   // the limit allows.
   inline bool excess_allocation();