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/api/BUILD.gn b/api/BUILD.gn
index d4cd733..e385c3c 100644
--- a/api/BUILD.gn
+++ b/api/BUILD.gn
@@ -425,6 +425,18 @@
     ]
   }
 
+  rtc_source_set("mock_video_bitrate_allocator") {
+    testonly = true
+    sources = [
+      "test/mock_video_bitrate_allocator.h",
+    ]
+
+    deps = [
+      "../api/video:video_bitrate_allocator",
+      "../test:test_support",
+    ]
+  }
+
   rtc_source_set("mock_video_codec_factory") {
     testonly = true
     sources = [