Fix LogLineRef for when logging is disabled.

No-Try: True
Bug: b/238157120
Change-Id: Id8dd096884ba5808c78ea93f1886ca259d4573ab
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/270484
Reviewed-by: Artem Titov <titovartem@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#37683}

rtc_base/logging.h

@@ -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;