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: https://chromium.googlesource.com/external/webrtc/+/5b4b52264132eefba10bc6880871715f9692da90

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-Original-Commit-Position: refs/heads/master@{#19742}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: a64685325c2f8f51873b67ae8a91f94ffb19d70b
7 files changed