Prepare for splitting the api/video:video_frames build rule.

This change is part of a change to break the dependency between "api:rtp_headers" and "api/video:video_frame". It does so by first creating an empty "api/video:video_rtp_headers" build rule so that downstream projects can be fixed before moving the source files.

Bug: webrtc:10668
Change-Id: I81aa6edfef3639b457a40aa93de048e62cbfd8ef
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/140291
Commit-Queue: Chen Xing <chxg@google.com>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#28209}
diff --git a/BUILD.gn b/BUILD.gn
index 4acda15..00a4932 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -427,7 +427,10 @@
     }
 
     if (build_with_mozilla) {
-      deps += [ "api/video:video_frame" ]
+      deps += [
+        "api/video:video_frame",
+        "api/video:video_rtp_headers",
+      ]
     } else {
       deps += [
         "api",