Update V8 to r6387 as required by WebKit r76408

Change-Id: Icfc5385b0996bd592f8b1ac8cbb44767ee09f1f6
diff --git a/src/preparse-data.h b/src/preparse-data.h
index cc82bcc..bb5707b 100644
--- a/src/preparse-data.h
+++ b/src/preparse-data.h
@@ -39,7 +39,7 @@
  public:
   // Layout and constants of the preparse data exchange format.
   static const unsigned kMagicNumber = 0xBadDead;
-  static const unsigned kCurrentVersion = 5;
+  static const unsigned kCurrentVersion = 6;
 
   static const int kMagicOffset = 0;
   static const int kVersionOffset = 1;