Let Call::OnRecoveredPacket parse RTP header extensions.

Packets recovered by ULPFEC enter through
RtpVideoStreamReceiver::OnRecoveredPacket, which does RTP
header extension parsing. Packets recovered by FlexFEC, however,
enter through Call::OnRecoveredPacket, which prior to this
CL did not do RTP header extension parsing.

The lack of RTP header extension parsing for FlexFEC packets is a
regression since https://codereview.webrtc.org/2886993005/.

TESTED=Using Android app with FlexFEC field trial enabled.
BUG=webrtc:5654

Review-Url: https://codereview.webrtc.org/3002023002
Cr-Commit-Position: refs/heads/master@{#19460}