Add header size both for network emulation and stats. Bug: webrtc:11003 Change-Id: I6f5b6bc1e761bdc40da4e2e0f10a9696e8a45c88 Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/155442 Commit-Queue: Niels Moller <nisse@webrtc.org> Reviewed-by: Sebastian Jansson <srte@webrtc.org> Cr-Commit-Position: refs/heads/master@{#29382}