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/video/BUILD.gn b/video/BUILD.gn
index 29e1e45..0ae719a 100644
--- a/video/BUILD.gn
+++ b/video/BUILD.gn
@@ -63,6 +63,7 @@
     "../api/video:video_bitrate_allocator",
     "../api/video:video_frame",
     "../api/video:video_frame_i420",
+    "../api/video:video_rtp_headers",
     "../api/video:video_stream_encoder",
     "../api/video:video_stream_encoder_create",
     "../api/video_codecs:video_codecs_api",
@@ -133,6 +134,7 @@
     "../api/task_queue",
     "../api/video:encoded_frame",
     "../api/video:video_frame",
+    "../api/video:video_rtp_headers",
     "../api/video:video_stream_decoder",
     "../api/video_codecs:video_codecs_api",
     "../modules/video_coding",
@@ -189,6 +191,7 @@
     "../api/video:video_bitrate_allocator_factory",
     "../api/video:video_frame",
     "../api/video:video_frame_i420",
+    "../api/video:video_rtp_headers",
     "../api/video:video_stream_encoder",
     "../api/video_codecs:video_codecs_api",
     "../common_video",
@@ -258,6 +261,7 @@
       "../api/video:builtin_video_bitrate_allocator_factory",
       "../api/video:video_bitrate_allocator_factory",
       "../api/video:video_frame",
+      "../api/video:video_rtp_headers",
       "../call:fake_network",
       "../call:simulated_network",
       "../common_video",
@@ -558,6 +562,7 @@
       "../api/video:video_frame",
       "../api/video:video_frame_i420",
       "../api/video:video_frame_type",
+      "../api/video:video_rtp_headers",
       "../api/video_codecs:video_codecs_api",
       "../api/video_codecs:vp8_temporal_layers_factory",
       "../call:call_interfaces",