Update V8 to r4851 as required by WebKit r61121

Change-Id: Ib01b7c8e38d5b82b254192fc06365aa5b85780c5
diff --git a/src/d8.cc b/src/d8.cc
index a69320a..7fd7925 100644
--- a/src/d8.cc
+++ b/src/d8.cc
@@ -576,6 +576,9 @@
 void Shell::RunShell() {
   LineEditor* editor = LineEditor::Get();
   printf("V8 version %s [console: %s]\n", V8::GetVersion(), editor->name());
+  if (i::FLAG_debugger) {
+    printf("JavaScript debugger enabled\n");
+  }
   editor->Open();
   while (true) {
     Locker locker;