This fixes a couple major issues.
#1: If the payload type that an RTX codec refers to has been reassigned, and then the RTX codec is added in a subsequent offer, it refers to the wrong payload type.
#2: If we receive an offer with two payload types referring to the same codec (which we support), our answer contains both (instead of just one), which causes issues down the road since the video engine only supports one payload type per codec.
BUG=webrtc:5450,webrtc:5499
R=pthatcher@webrtc.org
Review URL: https://codereview.webrtc.org/1616033002 .
Cr-Commit-Position: refs/heads/master@{#11880}