Update outdated comment.

Bug: None
No-Try: true
Change-Id: I5289920a1b59c84333374abbb271332bd7e6fd8b
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/374482
Commit-Queue: Jeremy Leconte <jleconte@webrtc.org>
Reviewed-by: Jeremy Leconte <jleconte@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#43879}

rtc_base/system/file_wrapper.h

@@ -87,7 +87,7 @@ class FileWrapper final {
   // Seek to given position.
   bool SeekTo(int64_t position);
 
-  // Returns the file size or -1 if a size could not be determined.
+  // Returns the file size or std::nullopt if the size could not be determined.
   // (A file size might not exists for non-seekable files or file-like
   // objects, for example /dev/tty on unix.)
   std::optional<size_t> FileSize();