Introduce an empty target for video_frame_writer

This target then will be filled with writer related part from "video_test_support"
This allows downstream to migrate on the new target keeping dependency on the old one.

Bug: None
Change-Id: Ie0b2f0ff9c7896c70b9a204ffedf15afac43c143
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/355580
Commit-Queue: Artem Titov <titovartem@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#42539}

test/BUILD.gn

@@ -490,6 +490,7 @@ rtc_library("fixed_fps_video_frame_writer_adapter") {
     "testsupport/fixed_fps_video_frame_writer_adapter.h",
   ]
   deps = [
+    ":video_frame_writer",
     ":video_test_support",
     "../api/test/video:video_frame_writer",
     "../api/units:time_delta",
@@ -500,6 +501,12 @@ rtc_library("fixed_fps_video_frame_writer_adapter") {
   ]
 }
 
+rtc_library("video_frame_writer") {
+  sources = []
+
+  deps = []
+}
+
 rtc_library("video_test_support") {
   testonly = true