Update V8 to r4851 as required by WebKit r61121

Change-Id: Ib01b7c8e38d5b82b254192fc06365aa5b85780c5
diff --git a/src/v8threads.cc b/src/v8threads.cc
index 02292f6..1e5e82e 100644
--- a/src/v8threads.cc
+++ b/src/v8threads.cc
@@ -331,7 +331,7 @@
 }
 
 
-void ThreadManager::IterateThreads(ThreadVisitor* v) {
+void ThreadManager::IterateArchivedThreads(ThreadVisitor* v) {
   for (ThreadState* state = ThreadState::FirstInUse();
        state != NULL;
        state = state->Next()) {