Introduce GN arg rtc_exclude_runtime_enabled_features_default.
Introduce GN arg rtc_exclude_runtime_enabled_features_default.

This GN argument will be used to exclude the default implementation of
runtime_features_enabled in order to allow clients to provide a custom
implementation.

This will allow to land [1] without breaking Chromium.

[1] - https://webrtc-review.googlesource.com/c/src/+/100640

Bug: webrtc:9631
Change-Id: I4ce8ff12e277f81de42e272d8874d5bb3a4a2635
Reviewed-on: https://webrtc-review.googlesource.com/100641
Reviewed-by: Karl Wiberg <2c708eb0a58047bf403399f50b3da33ffd2dd444@webrtc.org>
Commit-Queue: Mirko Bonadei <d2c43c210eae6feef04f53bae50885e8152edcca@webrtc.org>
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#24753}
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