message: Change device string in RecvNodeDescriptor in VirtualScheduler from const
  reference to const as the RecvNodeDescriptor (and cached_recv_nodes map)
  outlives device string from the NodeDef.
diff: @@ -62,7 +62,7 @@ Costs CombineCosts(const Costs& left, const Costs& right) {
  struct RecvNodeDescriptor {
  const NodeDef* node;
  const int port_num;
  - const string& device;
  + const string device;
  RecvNodeDescriptor(const NodeDef* node_, const int port_num_,
  const string& device_)
  