Add build argument rtc_exclude_system_time

This is the first CL out of three to enable overriding
of the function SystemTimeNanos() in
rtc_base/system_time.cc

Bug: chromium:516700
Change-Id: I7c33b0d3463fd68c777ef0c6d268dbde45746c64
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/208225
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Johannes Kron <kron@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#33303}

webrtc.gni

@@ -59,6 +59,12 @@ declare_args() {
   # provided.
   rtc_exclude_metrics_default = build_with_chromium
 
+  # Setting this to true will define WEBRTC_EXCLUDE_SYSTEM_TIME which
+  # will tell the pre-processor to remove the default definition of the
+  # SystemTimeNanos() which is defined in rtc_base/system_time.cc. In
+  # that case a new implementation needs to be provided.
+  rtc_exclude_system_time = 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.