Convert kMax constants in video_codec_constants to size_t

There was a pattern of having to cast these to size_t when comparing
them to sizes of things. Since these are mostly used for sizing
arrays and lists, it seems logical to make them size_t.

Bug: None
Change-Id: I77e0642dfc0937e6367034af10b27f1ba95b495b
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/459400
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Tomas Gunnarsson <tommi@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47281}