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/sdk/BUILD.gn b/sdk/BUILD.gn
index d389298..60be6db 100644
--- a/sdk/BUILD.gn
+++ b/sdk/BUILD.gn
@@ -332,6 +332,7 @@
         "../api:libjingle_peerconnection_api",
         "../api/video:video_frame",
         "../api/video:video_frame_i420",
+        "../api/video:video_rtp_headers",
         "../common_video",
         "../media:rtc_media_base",
         "../rtc_base",
@@ -360,6 +361,7 @@
         ":base_objc",
         "//api/video:video_frame",
         "//api/video:video_frame_i420",
+        "//api/video:video_rtp_headers",
         "//common_video",
         "//rtc_base:checks",
         "//rtc_base:rtc_base_approved",
@@ -413,6 +415,7 @@
         ":videosource_objc",
         "../api:libjingle_peerconnection_api",
         "../api/video:video_frame",
+        "../api/video:video_rtp_headers",
         "../common_video",
         "../media:rtc_media_base",
         "../rtc_base",
@@ -491,6 +494,7 @@
           ":video_objc",
           ":videoframebuffer_objc",
           "../api/video:video_frame",
+          "../api/video:video_rtp_headers",
           "../rtc_base:checks",
           "../rtc_base:rtc_base_approved",
         ]
@@ -915,6 +919,7 @@
         "../api/audio_codecs:builtin_audio_encoder_factory",
         "../api/task_queue:default_task_queue_factory",
         "../api/video:video_frame",
+        "../api/video:video_rtp_headers",
         "../api/video_codecs:video_codecs_api",
         "../common_video",
         "../logging:rtc_event_log_impl_base",
@@ -1551,6 +1556,7 @@
         "../api:libjingle_peerconnection_api",
         "../api:scoped_refptr",
         "../api/video:video_frame",
+        "../api/video:video_rtp_headers",
         "../api/video_codecs:video_codecs_api",
         "../common_video",
         "../rtc_base",
@@ -1588,6 +1594,7 @@
         ":wrapped_native_codec_objc",
         "../api/video:video_frame",
         "../api/video:video_frame_i420",
+        "../api/video:video_rtp_headers",
         "../api/video_codecs:video_codecs_api",
         "../common_video",
         "../media:rtc_audio_video",