Exposing video bitrate allocator into API

In order to have public video bitrate allocator factory, the video bitrate allocator has be part of
the api.

Bug: webrtc:9513
Change-Id: Ia2e5ab9eb988c710c1ac492afccc470a92544aa2
Reviewed-on: https://webrtc-review.googlesource.com/88083
Reviewed-by: Stefan Holmer <stefan@webrtc.org>
Reviewed-by: Rasmus Brandt <brandtr@webrtc.org>
Reviewed-by: Erik Språng <sprang@webrtc.org>
Commit-Queue: Jiawei Ou <ouj@fb.com>
Cr-Commit-Position: refs/heads/master@{#24073}
diff --git a/video/BUILD.gn b/video/BUILD.gn
index a135696..844941a 100644
--- a/video/BUILD.gn
+++ b/video/BUILD.gn
@@ -61,6 +61,7 @@
     "../api:fec_controller_api",
     "../api:libjingle_peerconnection_api",
     "../api:transport_api",
+    "../api/video:video_bitrate_allocator",
     "../api/video:video_frame",
     "../api/video:video_frame_i420",
     "../api/video:video_stream_encoder",