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/api/BUILD.gn b/api/BUILD.gn
index 063767a..9c35682 100644
--- a/api/BUILD.gn
+++ b/api/BUILD.gn
@@ -74,6 +74,7 @@
     ":array_view",
     "..:webrtc_common",
     "video:video_frame",
+    "video:video_rtp_headers",
     "//third_party/abseil-cpp/absl/types:optional",
   ]
 }
@@ -180,6 +181,7 @@
     "units:timestamp",
     "video:encoded_image",
     "video:video_frame",
+    "video:video_rtp_headers",
     "//third_party/abseil-cpp/absl/algorithm:container",
     "//third_party/abseil-cpp/absl/strings",
     "//third_party/abseil-cpp/absl/types:optional",
@@ -236,6 +238,7 @@
     ":stats_observer_interface",
     "video:encoded_image",
     "video:video_frame",
+    "video:video_rtp_headers",
     "video_codecs:video_codecs_api",
     "//third_party/abseil-cpp/absl/types:optional",
   ]