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 <kwiberg@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#24753}

webrtc.gni

@@ -54,6 +54,22 @@ declare_args() {
   # provided.
   rtc_exclude_metrics_default = build_with_chromium
 
+  # WARNING: This argument doesn't have any effect on the WebRTC build until
+  # https://webrtc-review.googlesource.com/c/src/+/100640 will land.
+  # Setting this to true will define
+  # WEBRTC_EXCLUDE_RUNTIME_ENABLED_FEATURES_DEFAULT which will tell the
+  # pre-processor to remove the default definition of symbols needed to use
+  # runtime_enabled_features. In that case a new implementation needs to be
+  # provided.
+  if (build_with_chromium) {
+    # When WebRTC is built as part of Chromium it should exclude the default
+    # implementation of runtime_enabled_features unless it is building for NACL or
+    # Chromecast.
+    rtc_exclude_runtime_enabled_features_default = !is_nacl && !is_chromecast
+  } else {
+    rtc_exclude_runtime_enabled_features_default = false
+  }
+
   # Setting this to false will require the API user to pass in their own
   # SSLCertificateVerifier to verify the certificates presented from a
   # TLS-TURN server. In return disabling this saves around 100kb in the binary.