Reduce VideoSendStream recreations due to FlexFEC.

This CL reduces the number of VideoSendStream recreations during SDP
renegotiation by checking the FlexFEC field trials before, and not after,
the SDP codec diffing logic.

BUG=webrtc:5654

Review-Url: https://codereview.webrtc.org/2882433003
Cr-Original-Commit-Position: refs/heads/master@{#18211}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 31bd224f356b6dff18664370abd475351de4c683
2 files changed