Call Device.SetCarrier when the ONC Carrier property is specified.

extensions/common/api/networking_private.idl

Index: extensions/common/api/networking_private.idl
diff --git a/extensions/common/api/networking_private.idl b/extensions/common/api/networking_private.idl
index e10583adafe87ba71ce4809b00c8bd51b9440b9c..3173fb0a3e4b899a32ad3f96b606883e6b8b3e35 100644
--- a/extensions/common/api/networking_private.idl
+++ b/extensions/common/api/networking_private.idl
@@ -60,6 +60,11 @@ namespace networkingPrivate {
   dictionary CellularConfigProperties {
     boolean? AutoConnect;
     APNProperties? APN;
+
+    // Specifies which carrier to use for Cellular configurations that support
+    // multiple carriers. May be set with $(ref:setProperties), but will be
+    // ignored by $(ref:createConfiguration).
+    DOMString? Carrier;
   };
 
   dictionary CellularStateProperties {