Update V8 to r6768 as required by WebKit r78450

Change-Id: Ib8868ff7147a76547a8d1d85f257ebe8546a3d3f
diff --git a/src/date.js b/src/date.js
index 9eb607c..1fb4897 100644
--- a/src/date.js
+++ b/src/date.js
@@ -605,7 +605,7 @@
 
 // ECMA 262 - 15.9.5.5
 function DateToLocaleString() {
-  return DateToString.call(this);
+  return %_CallFunction(this, DateToString);
 }
 
 
@@ -973,7 +973,7 @@
 // do that either.  Instead, we create a new function whose name
 // property will return toGMTString.
 function DateToGMTString() {
-  return DateToUTCString.call(this);
+  return %_CallFunction(this, DateToUTCString);
 }