Reverse |rtx_payload_types| map, and rename.

New name is |rtx_associated_payload_types|.

BUG=webrtc:7135

Review-Url: https://codereview.webrtc.org/3000273002
Cr-Original-Commit-Position: refs/heads/master@{#19514}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 26e3abbb40d5fc24642ed61ca226810713084dec
10 files changed