Implemented really weak handles.

src/api.cc

Index: src/api.cc
diff --git a/src/api.cc b/src/api.cc
index 00f1e0b7e156dd3f4a8810ddcb90f580450cefc0..846ebf3de6beea794a144a10bd0e180bf7d44641 100644
--- a/src/api.cc
+++ b/src/api.cc
@@ -372,6 +372,13 @@ void V8::MakeWeak(i::Object** object, void* parameters,
 }
 
 
+void V8::MakeReallyWeak(i::Object** object, void* parameters,
+                        WeakReferenceCallback callback) {
+  LOG_API("MakeReallyWeak");
+  i::GlobalHandles::MakeReallyWeak(object, parameters, callback);
+}
+
+
 void V8::ClearWeak(i::Object** obj) {
   LOG_API("ClearWeak");
   i::GlobalHandles::ClearWeakness(obj);
@@ -395,7 +402,9 @@ bool V8::IsGlobalWeak(i::Object** obj) {
 void V8::DisposeGlobal(i::Object** obj) {
   LOG_API("DisposeGlobal");
   if (!i::V8::IsRunning()) return;
-  if ((*obj)->IsGlobalContext()) i::Heap::NotifyContextDisposed();
+  if (!i::GlobalHandles::IsReallyWeak(obj) && (*obj)->IsGlobalContext()) {
+    i::Heap::NotifyContextDisposed();
+  }
   i::GlobalHandles::Destroy(obj);
 }