Sketch out detaching threads (with partial implementation).

Change-Id: Iafe5f91c8170d4f90155f509ba9e2e3d921af66f
diff --git a/src/runtime.cc b/src/runtime.cc
index c8f9036..2acf2ec 100644
--- a/src/runtime.cc
+++ b/src/runtime.cc
@@ -472,9 +472,7 @@
 }
 
 void Runtime::DetachCurrentThread() {
-  Thread* self = Thread::Current();
-  thread_list_->Unregister(self);
-  delete self;
+  thread_list_->Unregister();
 }
 
 void Runtime::VisitRoots(Heap::RootVisitor* visitor, void* arg) const {