Reland Use runtime enabled features API to enable dual stream mode

This is an unchanged patch after dependency fixes in downstream projects are implemented.

Original patch was reviewed here:
https://webrtc-review.googlesource.com/c/src/+/39008

TBR=phoglund@webrtc.org,ilnik@webrtc.org,nisse@webrtc.org,philipel@webrtc.org,lliuu@webrtc.org

Change-Id: I648bbf63d34282a48cabc854615005ec65b28cb3
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: webrtc:8287
Reviewed-on: https://webrtc-review.googlesource.com/40420
Commit-Queue: Ilya Nikolaevskiy <ilnik@webrtc.org>
Reviewed-by: Ilya Nikolaevskiy <ilnik@webrtc.org>
Reviewed-by: Niels Moller <nisse@webrtc.org>
Reviewed-by: Patrik Höglund <phoglund@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#21674}
diff --git a/system_wrappers/source/runtime_enabled_features_default.cc b/system_wrappers/source/runtime_enabled_features_default.cc
index cd0d157..31a3ff7 100644
--- a/system_wrappers/source/runtime_enabled_features_default.cc
+++ b/system_wrappers/source/runtime_enabled_features_default.cc
@@ -22,7 +22,7 @@
 namespace runtime_enabled_features {
 
 bool IsFeatureEnabled(std::string feature_name) {
-  if (feature_name == "WebRtcDualStreamMode")
+  if (feature_name == kDualStreamModeFeatureName)
     return flags::FLAG_enable_dual_stream_mode;
   return false;
 }