message: Include what you use fixes.
  Relying on transitive dependencies for what you are going to use is a bad idea because it can break on changes done by said transitive dependencies.
  This CL adds necessary headers for accessing gRPC.
diff: @@ -18,13 +18,10 @@ limitations under the License.
  #include <memory>
  +#include "grpcpp/completion_queue.h"
  #include "tensorflow/core/distributed_runtime/rpc/grpc_util.h"
  #include "tensorflow/core/lib/core/threadpool.h"
  -namespace grpc {
  -class CompletionQueue;
  -}
  -
  namespace tensorflow {
  class WorkerCacheLogger;
  class WorkerInterface;
  