Use runtime enabled features API to enable dual stream mode

Bug: webrtc:8287
Change-Id: I1a366d959a8b7f2a704baa7ea8ace64c1c398d52
Reviewed-on: https://webrtc-review.googlesource.com/39008
Commit-Queue: Ilya Nikolaevskiy <ilnik@webrtc.org>
Reviewed-by: Niels Moller <nisse@webrtc.org>
Reviewed-by: Philip Eliasson <philipel@webrtc.org>
Reviewed-by: Patrik Höglund <phoglund@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#21661}

system_wrappers/include/runtime_enabled_features.h

@@ -21,8 +21,12 @@
 // TODO(ilnik): Find a more flexible way to use Chrome features.
 // This interface requires manual translation from feature name to
 // Chrome feature class in third_party/webrtc_overrides.
+
 namespace webrtc {
 namespace runtime_enabled_features {
+
+const char kDualStreamModeFeatureName[] = "WebRtcDualStreamMode";
+
 bool IsFeatureEnabled(std::string feature_name);
 
 }  // namespace runtime_enabled_features