Add a convenience method to get the debugged context

src/debug/debug.cc

Index: src/debug/debug.cc
diff --git a/src/debug/debug.cc b/src/debug/debug.cc
index 495e162f28194ce34034dcdf5139934728642a9d..519d456b9fe9b81b2846a642759c0a4a17edf7cb 100644
--- a/src/debug/debug.cc
+++ b/src/debug/debug.cc
@@ -2493,6 +2493,9 @@ v8::Debug::ClientData* EventDetailsImpl::GetClientData() const {
   return client_data_;
 }
 
+v8::Isolate* EventDetailsImpl::GetIsolate() const {
+  return reinterpret_cast<v8::Isolate*>(exec_state_->GetIsolate());
+}
 
 CommandMessage::CommandMessage() : text_(Vector<uint16_t>::empty()),
                                    client_data_(NULL) {