Fix availble new space memory statistics reporting.

src/spaces.h

Index: src/spaces.h
===================================================================
--- src/spaces.h	(revision 9590)
+++ src/spaces.h	(working copy)
@@ -2100,10 +2100,9 @@
     return Capacity();
   }
 
-  // Return the available bytes without growing or switching page in the
-  // active semispace.
+  // Return the available bytes without growing.
   intptr_t Available() {
-    return allocation_info_.limit - allocation_info_.top;
+    return Capacity() - Size();
   }
 
   // Return the maximum capacity of a semispace.