Update V8 to r6768 as required by WebKit r78450

Change-Id: Ib8868ff7147a76547a8d1d85f257ebe8546a3d3f
diff --git a/src/hashmap.h b/src/hashmap.h
index 3b947be..2798988 100644
--- a/src/hashmap.h
+++ b/src/hashmap.h
@@ -49,7 +49,8 @@
   typedef bool (*MatchFun) (void* key1, void* key2);
 
   // Dummy constructor.  This constructor doesn't set up the hash
-  // map properly so don't use it unless you have good reason.
+  // map properly so don't use it unless you have good reason (e.g.,
+  // you know that the HashMap will never be used).
   HashMap();
 
   // initial_capacity is the size of the initial hash map;