Re-land: "Use an explicit identifier in Config"

This let's us use them to configure them when using WebRTC as an external library. One use case where this is necessary is in the Android OS.

Original CL: https://codereview.webrtc.org/1538643004/

TBR=tommi@webrtc.org

Review URL: https://codereview.webrtc.org/1589573004

Cr-Original-Commit-Position: refs/heads/master@{#11248}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 688e308a353c27803a66803230235638f4dd1f2b
diff --git a/config.h b/config.h
index 45a2ece..06460ae 100644
--- a/config.h
+++ b/config.h
@@ -16,6 +16,7 @@
 #include <string>
 #include <vector>
 
+#include "webrtc/common.h"
 #include "webrtc/common_types.h"
 #include "webrtc/typedefs.h"
 
@@ -128,6 +129,7 @@
 struct NetEqCapacityConfig {
   NetEqCapacityConfig() : enabled(false), capacity(0) {}
   explicit NetEqCapacityConfig(int value) : enabled(true), capacity(value) {}
+  static const ConfigOptionID identifier = ConfigOptionID::kNetEqCapacityConfig;
   bool enabled;
   int capacity;
 };
@@ -135,12 +137,14 @@
 struct NetEqFastAccelerate {
   NetEqFastAccelerate() : enabled(false) {}
   explicit NetEqFastAccelerate(bool value) : enabled(value) {}
+  static const ConfigOptionID identifier = ConfigOptionID::kNetEqFastAccelerate;
   bool enabled;
 };
 
 struct VoicePacing {
   VoicePacing() : enabled(false) {}
   explicit VoicePacing(bool value) : enabled(value) {}
+  static const ConfigOptionID identifier = ConfigOptionID::kVoicePacing;
   bool enabled;
 };