[Isolates] More handle improvements:

src/handles.h

Index: src/handles.h
diff --git a/src/handles.h b/src/handles.h
index 254a0de043553bf515ffcc536eccde152a659474..17392878462f8788adff9b2d57ed046566412e5c 100644
--- a/src/handles.h
+++ b/src/handles.h
@@ -44,6 +44,7 @@ class Handle {
  public:
   INLINE(explicit Handle(T** location)) { location_ = location; }
   INLINE(explicit Handle(T* obj));
+  INLINE(explicit Handle(HeapObject* obj));
   INLINE(Handle(T* obj, Isolate* isolate));
 
   INLINE(Handle()) : location_(NULL) {}
@@ -109,6 +110,7 @@ class Handle {
 class HandleScope {
  public:
   inline HandleScope();
+  explicit inline HandleScope(Isolate* isolate);
 
   ~HandleScope() {
     Leave(&previous_);