Update V8 to r3431 as required by WebKit r51976.

Change-Id: I567392c3f8c0a0d5201a4249611ac4ccf468cd5b
diff --git a/src/zone.h b/src/zone.h
index 4e4f1d7..0d006dd 100644
--- a/src/zone.h
+++ b/src/zone.h
@@ -118,7 +118,7 @@
 class ZoneObject {
  public:
   // Allocate a new ZoneObject of 'size' bytes in the Zone.
-  void* operator new(size_t size) { return Zone::New(size); }
+  void* operator new(size_t size) { return Zone::New(static_cast<int>(size)); }
 
   // Ideally, the delete operator should be private instead of
   // public, but unfortunately the compiler sometimes synthesizes