Added in-session captive portal notification.

ash/system/system_notifier.cc

Index: ash/system/system_notifier.cc
diff --git a/ash/system/system_notifier.cc b/ash/system/system_notifier.cc
index e4d65d27338fcfac3be006d1f8707f3968cc5387..8c4b1604b8617cab1156ccfc81c033e5b16728ae 100644
--- a/ash/system/system_notifier.cc
+++ b/ash/system/system_notifier.cc
@@ -30,6 +30,7 @@ const char* kAshSystemNotifiers[] = {
   kNotifierMultiProfileFirstRun,
   kNotifierNetwork,
   kNotifierNetworkError,
+  kNotifierNetworkPortalDetector,
   kNotifierScreenshot,
   kNotifierScreenCapture,
   kNotifierScreenShare,
@@ -61,6 +62,7 @@ const char kNotifierLocallyManagedUser[] = "ash.locally-managed-user";
 const char kNotifierMultiProfileFirstRun[] = "ash.multi-profile.first-run";
 const char kNotifierNetwork[] = "ash.network";
 const char kNotifierNetworkError[] = "ash.network.error";
+const char kNotifierNetworkPortalDetector[] = "ash.network.portal-detector";
 const char kNotifierScreenshot[] = "ash.screenshot";
 const char kNotifierScreenCapture[] = "ash.screen-capture";
 const char kNotifierScreenShare[] = "ash.screen-share";