Reason for revert:
The cl this change depended on has now been successfully relanded.
Original issue's description:
> Revert of Delete Rtx-related methods from RTPPayloadRegistry. (patchset #3 id:40001 of https://codereview.webrtc.org/3006993002/ )
>
> Reason for revert:
> This has to be reverted to enable reverting cl https://codereview.webrtc.org/3006063002/, which seems to have broken ulpfec.
>
> Original issue's description:
> > Delete Rtx-related methods from RTPPayloadRegistry.
> >
> > Delete methods IsRtx, IsEncapsulated and RestoreOriginalPacket.
> >
> > BUG=webrtc:7135
> >
> > Review-Url: https://codereview.webrtc.org/3006993002
> > Cr-Commit-Position: refs/heads/master@{#19739}
> > Committed: 5b4b522641
>
> TBR=stefan@webrtc.org,danilchap@webrtc.org
> # Skipping CQ checks because original CL landed less than 1 days ago.
> NOPRESUBMIT=true
> NOTREECHECKS=true
> NOTRY=true
> BUG=webrtc:7135
>
> Review-Url: https://codereview.webrtc.org/3011093002
> Cr-Commit-Position: refs/heads/master@{#19742}
> Committed: a64685325cTBR=stefan@webrtc.org,danilchap@webrtc.org
# Not skipping CQ checks because original CL landed more than 1 days ago.
BUG=webrtc:7135
Review-Url: https://codereview.webrtc.org/3012253002
Cr-Commit-Position: refs/heads/master@{#19821}
f54573bd3b