message: Include what you use fix.
  The ::grpc::Channel definition used in the rest of the file is currently being transitively imported from the generated protobuf code. Let's include the file explicitely instead.
diff: @@ -253,6 +253,7 @@ struct hash<::tensorflow::DebugNodeKey> {
  // TODO(cais): Support grpc:// debug URLs in open source once Python grpc
  // genrule becomes available. See b/23796275.
  #ifndef PLATFORM_WINDOWS
  +#include "grpcpp/channel.h"
  #include "tensorflow/core/debug/debug_service.grpc.pb.h"
  namespace tensorflow {
  