Added ClearTo(seq_num) to RtpFrameReferenceFinder.

In order to be able to clear out any potentially stashed old frames from
the RtpFrameReferenceFinder we can now clear frames that contain packets
older than |seq_num|.

BUG=webrtc:5514

Review-Url: https://codereview.webrtc.org/2304723004
Cr-Original-Commit-Position: refs/heads/master@{#14156}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 463d3011ceeb6d361411a42fa974b92d9dbc53e1
3 files changed