Update transport overhead the first time the network connects.
Update transport overhead the first time the network connects.

This fixes a bug where transport_overhead_bytes_per_packet_ is sometimes
not set and therefore not included in the BWE.

Bug: webrtc:11359
Change-Id: Id3593299c6bcd7ce3c44a7b148c202240b3f1981
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/168525
Reviewed-by: Christoffer Rodbro <a8869a0b8551100ac7ab874b7705be1f60ffd43b@webrtc.org>
Reviewed-by: Sebastian Jansson <c24d1bc144eb7c11e553f59153fa17dba85ca83e@webrtc.org>
Commit-Queue: Jakob Ivarsson <9ca81c2aa4aa86256fc426d7d57eed586c9fd3ac@webrtc.org>
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#30522}

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