message: 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
  Cr-Commit-Position: refs/heads/master@{#33303}
diff: @@ -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.
  