Calculate max payload size for an rtp packet to fit full video frame
Calculate max payload size for an rtp packet to fit full video frame

instead of sometimes incorrectly guessing it

Bug: webrtc:9868
Change-Id: I8b15ecca4c660d83ea129dc9df6ec174ad83b4c6
Reviewed-on: https://webrtc-review.googlesource.com/c/106281
Commit-Queue: Danil Chapovalov <55de8d9c2b5977da2dd48f967f21f9d606b172a5@webrtc.org>
Reviewed-by: Ilya Nikolaevskiy <76788ac3efd229e9d156135d9ceb7fef65c45da4@webrtc.org>
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#25213}
ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc