Refactor webrtc/modules/pacing for GN check

This moves some GN check configurations out of .gn to individual
targets.

The now checked target is:
"//webrtc/modules/pacing/*"

BUG=webrtc:6828
NOTRY=True

Review-Url: https://codereview.webrtc.org/2594523003
Cr-Commit-Position: refs/heads/master@{#15729}

.gn

@@ -30,6 +30,7 @@ check_targets = [
   "//webrtc/modules/audio_processing/*",
   "//webrtc/modules/desktop_capture/*",
   "//webrtc/modules/media_file/*",
+  "//webrtc/modules/pacing/*",
   "//webrtc/modules/video_capture/*",
   "//webrtc/modules/video_coding/*",
   "//webrtc/stats:rtc_stats",

webrtc/modules/pacing/BUILD.gn

@@ -26,6 +26,8 @@ rtc_static_library("pacing") {
   }
 
   deps = [
+    "../..:webrtc_common",
+    "../../base:rtc_base_approved",
     "../../system_wrappers",
     "../bitrate_controller",
     "../rtp_rtcp",