Make sure rtp_rtcp module doesn't directly reference anything in the pacer module, and remove build dependencies on it.

BUG=

Review URL: https://codereview.webrtc.org/1350163005

Cr-Commit-Position: refs/heads/master@{#10005}
diff --git a/webrtc/video/BUILD.gn b/webrtc/video/BUILD.gn
index 1d26b41..04d95ce 100644
--- a/webrtc/video/BUILD.gn
+++ b/webrtc/video/BUILD.gn
@@ -70,6 +70,7 @@
     "..:webrtc_common",
     "../common_video",
     "../modules/bitrate_controller",
+    "../modules/pacing",
     "../modules/rtp_rtcp",
     "../modules/utility",
     "../modules/video_capture:video_capture_module",