message: Implement WindowsWritableFile::Tell
  it was probably an oversight, as we were hurrying windows support out the door.
diff: @@ -174,6 +174,22 @@ class WindowsWritableFile : public WritableFile {
  return Status::OK();
  }
  + Status Tell(int64* position) override {
  + Status result = Flush();
  + if (!result.ok()) {
  + return result;
  + }
  +
  + *position = SetFilePointer(hfile_, 0, NULL, FILE_CURRENT);
  +
  + if (*position == INVALID_SET_FILE_POINTER) {
  + return IOErrorFromWindowsError(
  + "Tell(SetFilePointer) failed for: " + filename_, ::GetLastError());
  + }
  +
  + return Status::OK();
  + }
  +
  Status Close() override {
  assert(INVALID_HANDLE_VALUE != hfile_);
  