Refactor webrtc/modules/audio_conference_mixer for GN check

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

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

BUG=webrtc:6828
NOTRY=True

Review-Url: https://codereview.webrtc.org/2593003002
Cr-Commit-Position: refs/heads/master@{#15759}

.gn

@@ -25,6 +25,7 @@ check_targets = [
   "//webrtc/api/*",
   "//webrtc/audio/*",
   "//webrtc/modules/audio_coding/*",
+  "//webrtc/modules/audio_conference_mixer/*",
   "//webrtc/modules/audio_device/*",
   "//webrtc/modules/audio_mixer/*",
   "//webrtc/modules/audio_processing/*",

webrtc/modules/audio_conference_mixer/BUILD.gn

@@ -39,7 +39,9 @@ rtc_static_library("audio_conference_mixer") {
   }
 
   deps = [
+    "../..:webrtc_common",
     "../../audio/utility:audio_frame_operations",
+    "../../base:rtc_base_approved",
     "../../system_wrappers",
     "../audio_processing",
   ]