Revert "Remove deprecated video_frame_i420 build target"

This reverts commit 33bd4fbe1e5b0ddf350409431ee5d9bb4e2a8132.

Reason for revert: Breaks downstream project

Original change's description:
> Remove deprecated video_frame_i420 build target
>
> Bug: chromium:1132299
> Change-Id: Ie0207037b46d4c40d2d808bf5fcddf71f43e325b
> Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/187350
> Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
> Commit-Queue: Ilya Nikolaevskiy <ilnik@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#32385}

TBR=mbonadei@webrtc.org,ilnik@webrtc.org,mbonadei@chromium.org

Change-Id: I83185a905191877c5b8e9826cc3555fa9406a537
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: chromium:1132299
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/187496
Reviewed-by: Ilya Nikolaevskiy <ilnik@webrtc.org>
Commit-Queue: Ilya Nikolaevskiy <ilnik@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#32386}
diff --git a/api/video/BUILD.gn b/api/video/BUILD.gn
index d75ec8f..f4069d2 100644
--- a/api/video/BUILD.gn
+++ b/api/video/BUILD.gn
@@ -74,6 +74,13 @@
   }
 }
 
+# Deprecated empty target. Use "video_frame" instead.
+rtc_source_set("video_frame_i420") {
+  visibility = [ "*" ]
+  sources = []
+  deps = [ ":video_frame" ]
+}
+
 rtc_library("video_frame_i010") {
   visibility = [ "*" ]
   sources = [