Refactor webrtc/modules/pacing for GN check
Refactor webrtc/modules/pacing for GN check

This moves some GN check configurations out of .gn to individual
targets.

The now checked target is:
"//webrtc/modules/pacing/*"

BUG=webrtc:6828
NOTRY=True

Review-Url: https://codereview.webrtc.org/2594523003
Cr-Original-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#15729}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 2a495ca2971282bda060d603e95a498dd9839689

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