message: Add placeholder target to move rtc_error out of the main API target.
  No-Try: True
  Cr-Commit-Position: refs/heads/master@{#29127}
diff: @@ -204,6 +204,12 @@ rtc_static_library("libjingle_peerconnection_api") {
  ]
  }
  +rtc_source_set("rtc_error") {
  + visibility = [ "*" ]
  + # Placeholder target for the migration of rtc_error.{h,cc} out of
  + # libjingle_peerconnection_api.
  +}
  +
  rtc_source_set("packet_socket_factory") {
  visibility = [ "*" ]
  sources = [
  