Unwrap picture ids in the RtpFrameReferencerFinder.

First CL to avoid working with wrapping picture ids. After the references of
a frame has been determined they are then unwrapped.

BUG=webrtc:7874

Review-Url: https://codereview.webrtc.org/2985283002
Cr-Original-Commit-Position: refs/heads/master@{#19663}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: d4fac6957e14068f8dbf2ab4a20006b1df044274
9 files changed