message: internally (platform/google), we return thread name as <threadname>/<thread_id>
  externally (platform/default), we return thread name as <threadname>
  this cl make it consistent.
  seems that only consumer is TraceMeRecorder, which call this function once per thread per profile session. it seems okay for me to be a little bit slow.
diff: @@ -39,6 +39,7 @@ limitations under the License.
  #include "tensorflow/core/platform/logging.h"
  #include "tensorflow/core/platform/mutex.h"
  #include "tensorflow/core/platform/ram_file_system.h"
  +#include "tensorflow/core/platform/strcat.h"
  #include "tensorflow/core/protobuf/error_codes.pb.h"
  namespace tensorflow {
  @@ -151,7 +152,7 @@ class PosixEnv : public Env {
  auto thread_name =
  GetThreadNameRegistry().find(std::this_thread::get_id());
  if (thread_name != GetThreadNameRegistry().end()) {
  - *name = thread_name->second;
  + *name = strings::StrCat(thread_name->second, "/", GetCurrentThreadId());
  return true;
  }
  }
  