Enable automatic resizing for RTX-enabled senders.

These were accidentally disabled due to checking ssrcs_.size() (which
includes RTX SSRCs) instead of rtp.ssrcs.size() to determine whether a
stream is simulcast or not.

BUG=webrtc:4965
R=asapersson@webrtc.org

Review URL: https://codereview.webrtc.org/1318193003 .

Cr-Commit-Position: refs/heads/master@{#9907}
2 files changed