PacketBuffer now can save how many times a packet has been nacked. Also save size/max nack count in the FrameObject/RtpFrameObject. BUG=webrtc:5514 R=stefan@webrtc.org Review URL: https://codereview.webrtc.org/1988653002 . Cr-Original-Commit-Position: refs/heads/master@{#12863} Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc Cr-Mirrored-Commit: 5ceaaae3689cac9ae10dc7caf5802e8d6e0c7a99