Make SendStatisticsProxy let through FlexFEC packets.
Make SendStatisticsProxy let through FlexFEC packets.

This is initial work to get the stats working for FlexFEC.

BUG=webrtc:5654

Review-Url: https://codereview.webrtc.org/2498393002
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#15089}

TimothyGu/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc