Don't add FEC and RTX overheads when calculating a padding packet's maximum payload size.
Don't add FEC and RTX overheads when calculating a padding packet's maximum payload size.

BUG=webrtc:7433

Review-Url: https://codereview.webrtc.org/2892583002
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#18184}

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