Reland of Make rtcp parsing implementation private in RtcpReceiver (patchset #1 id:1 of https://codereview.webrtc.org/2332673003/ )
Reland of Make rtcp parsing implementation private in RtcpReceiver (patchset #1 id:1 of https://codereview.webrtc.org/2332673003/ )

Reason for revert:
Fuzzer changed not use functions moved to private.

Original issue's description:
> Revert of Make rtcp parsing implementation private in RtcpReceiver (patchset #1 id:1 of https://codereview.webrtc.org/2320603002/ )
>
> Reason for revert:
> Breaks fuzzer compilation.
>
> Original issue's description:
> > Make rtcp parsing implementation private in RtcpReceiver:
> > Function just for Parse and for Callbacks moved to private section.
> > All handles moved from protected to private section.
> >
> > BUG=webrtc:5260
> > R=sprang@webrtc.org
> >
> > Committed: https://crrev.com/faf708e238c7b43a732fbebf79ac9298b4b95a95
> > Cr-Commit-Position: refs/heads/master@{#14181}
>
> TBR=sprang@webrtc.org
> # Skipping CQ checks because original CL landed less than 1 days ago.
> NOPRESUBMIT=true
> NOTREECHECKS=true
> NOTRY=true
> BUG=webrtc:5260
>
> Committed: https://crrev.com/180e4525ca7c9a23602cdf37a8756df7d23e7143
> Cr-Commit-Position: refs/heads/master@{#14182}

TBR=sprang@webrtc.org
# Not skipping CQ checks because original CL landed more than 1 days ago.
BUG=webrtc:5260

Review-Url: https://codereview.webrtc.org/2336213002
Cr-Original-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#14200}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: dd12892ede7139f17355ec4e3a64b3382fcd6f28

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