Add new empty build targets rtp_rtcp_legacy and video_legacy.

Initial step to be able to land
https://webrtc-review.googlesource.com/c/src/+/196904

Bug: webrtc:11581
Change-Id: Iaab52e98f4562f701cf02e3f641b7b02a11b799e
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/197944
Reviewed-by: Erik Språng <sprang@webrtc.org>
Commit-Queue: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#32971}

modules/rtp_rtcp/BUILD.gn

@@ -319,6 +319,11 @@ rtc_library("rtp_rtcp") {
   ]
 }
 
+rtc_source_set("rtp_rtcp_legacy") {
+  # TODO(bugs.webrtc.org/11581): The files "source/rtp_rtcp_impl.cc"
+  # and "source/rtp_rtcp_impl.h" should be moved to this target.
+}
+
 rtc_library("rtcp_transceiver") {
   visibility = [ "*" ]
   public = [
@@ -523,6 +528,7 @@ if (rtc_include_tests) {
       ":rtcp_transceiver",
       ":rtp_rtcp",
       ":rtp_rtcp_format",
+      ":rtp_rtcp_legacy",
       "../../api:array_view",
       "../../api:libjingle_peerconnection_api",
       "../../api:mock_frame_encryptor",