message: Fix const correctness.
  Cr-Commit-Position: refs/heads/main@{#37679}
diff: @@ -111,12 +111,12 @@ class LogMessage;
  // the most flexible way.
  class LogLineRef {
  public:
  - absl::string_view message() { return message_; }
  - absl::string_view filename() { return filename_; };
  - int line() { return line_; };
  - absl::optional<PlatformThreadId> thread_id() { return thread_id_; };
  - webrtc::Timestamp timestamp() { return timestamp_; };
  - absl::string_view tag() const { return tag_; };
  + absl::string_view message() const { return message_; }
  + absl::string_view filename() const { return filename_; }
  + int line() const { return line_; }
  + absl::optional<PlatformThreadId> thread_id() const { return thread_id_; }
  + webrtc::Timestamp timestamp() const { return timestamp_; }
  + absl::string_view tag() const { return tag_; }
  LoggingSeverity severity() const { return severity_; }
  std::string DefaultLogLine() const;
  