Using signed integers allows to centralize checking of edge cases in RtpPacketizer::SplitAboutEqually and reduce chance of hitting issues with size_t underflow Bug: webrtc:9680 Change-Id: Ic05bf0a9565a277c4608f43061ca46cf44e82d08 Reviewed-on: https://webrtc-review.googlesource.com/98602 Commit-Queue: Danil Chapovalov <danilchap@webrtc.org> Reviewed-by: Ilya Nikolaevskiy <ilnik@webrtc.org> Cr-Commit-Position: refs/heads/master@{#24618}