message: Fix LogLineRef for when logging is disabled.
  No-Try: True
  Cr-Commit-Position: refs/heads/main@{#37683}
diff: @@ -119,7 +119,11 @@ class LogLineRef {
  absl::string_view tag() const { return tag_; }
  LoggingSeverity severity() const { return severity_; }
  +#if RTC_LOG_ENABLED()
  std::string DefaultLogLine() const;
  +#else
  + std::string DefaultLogLine() const { return ""; }
  +#endif
  private:
  friend class LogMessage;
  