8-byte align zone allocations of objects that may require it.

src/zone.h

Index: src/zone.h
===================================================================
--- src/zone.h	(revision 9962)
+++ src/zone.h	(working copy)
@@ -86,7 +86,9 @@
   friend class Isolate;
   friend class ZoneScope;
 
-  // All pointers returned from New() have this alignment.
+  // All pointers returned from New() have this alignment.  In addition, if the
+  // object being allocated has a size that is divisible by 8 then its alignment
+  // will be 8.
   static const int kAlignment = kPointerSize;
 
   // Never allocate segments smaller than this size in bytes.