Update V8 to r5901 as required by WebKit r73109

Change-Id: Ic48c5b085ce90e0151e2e7e58c4c5afe87fce9d1
diff --git a/src/bootstrapper.cc b/src/bootstrapper.cc
index 0e49966..f60a975 100644
--- a/src/bootstrapper.cc
+++ b/src/bootstrapper.cc
@@ -39,6 +39,8 @@
 #include "objects-visiting.h"
 #include "snapshot.h"
 #include "stub-cache.h"
+#include "extensions/externalize-string-extension.h"
+#include "extensions/gc-extension.h"
 
 namespace v8 {
 namespace internal {
@@ -137,6 +139,8 @@
 
 void Bootstrapper::Initialize(bool create_heap_objects) {
   extensions_cache.Initialize(create_heap_objects);
+  GCExtension::Register();
+  ExternalizeStringExtension::Register();
 }