GN: Make audio_processing_unittests compile with rtc_enable_protobuf=false

BUG=webrtc:6626
NOTRY=True

Review-Url: https://codereview.webrtc.org/2647003002
Cr-Commit-Position: refs/heads/master@{#16209}
diff --git a/webrtc/BUILD.gn b/webrtc/BUILD.gn
index eb6c3f7..5633f28 100644
--- a/webrtc/BUILD.gn
+++ b/webrtc/BUILD.gn
@@ -371,6 +371,7 @@
       "base:rtc_task_queue_unittests",
       "p2p:libstunprober_unittests",
       "p2p:rtc_p2p_unittests",
+      "system_wrappers:metrics_default",
     ]
 
     if (rtc_enable_protobuf) {