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}
diff --git a/modules/rtp_rtcp/BUILD.gn b/modules/rtp_rtcp/BUILD.gn
index 100a338..e10d846 100644
--- a/modules/rtp_rtcp/BUILD.gn
+++ b/modules/rtp_rtcp/BUILD.gn
@@ -319,6 +319,11 @@
   ]
 }
 
+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 @@
       ":rtcp_transceiver",
       ":rtp_rtcp",
       ":rtp_rtcp_format",
+      ":rtp_rtcp_legacy",
       "../../api:array_view",
       "../../api:libjingle_peerconnection_api",
       "../../api:mock_frame_encryptor",
diff --git a/video/BUILD.gn b/video/BUILD.gn
index 8289023..acd1616 100644
--- a/video/BUILD.gn
+++ b/video/BUILD.gn
@@ -154,6 +154,25 @@
   }
 }
 
+rtc_source_set("video_legacy") {
+  # TODO(bugs.webrtc.org/11581): These files should be moved to this target:
+  #
+  # "call_stats.cc",
+  # "call_stats.h",
+  # "receive_statistics_proxy.cc",
+  # "receive_statistics_proxy.h",
+  # "rtp_streams_synchronizer.cc",
+  # "rtp_streams_synchronizer.h",
+  # "rtp_video_stream_receiver.cc",
+  # "rtp_video_stream_receiver.h",
+  # "video_quality_observer.cc",
+  # "video_quality_observer.h",
+  # "video_receive_stream.cc",
+  # "video_receive_stream.h",
+  # "video_stream_decoder.cc",
+  # "video_stream_decoder.h",
+}
+
 rtc_library("video_stream_decoder_impl") {
   visibility = [ "*" ]
 
@@ -571,6 +590,7 @@
     ]
     deps = [
       ":video",
+      ":video_legacy",
       ":video_mocks",
       ":video_stream_decoder_impl",
       ":video_stream_encoder_impl",