tray: Update network list to show nothing after toggle wifi on and off.

The wifi disabled message would show on toggle wifi off, but remain
when toggling wifi back on.

Test: manual
Bug: 823236
Change-Id: Ib232a917e45fea83a72792d0fbf30374661477d2
Reviewed-on: https://chromium-review.googlesource.com/1102896Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Reviewed-by: Xiaoyin Hu <xiaoyinh@chromium.org>
Commit-Queue: Sammie Quon <sammiequon@chromium.org>
Cr-Commit-Position: refs/heads/master@{#567801}

ash/system/network/network_list.cc

@@ -703,10 +703,12 @@ NetworkListView::UpdateNetworkListEntries() {
     return new_guids;
   }
 
+  bool should_clear_info_label = true;
   if (!handler->FirstNetworkByType(NetworkTypePattern::WiFi())) {
     UpdateInfoLabel(IDS_ASH_STATUS_TRAY_NETWORK_WIFI_ENABLED, index,
                     &no_wifi_networks_view_);
     ++index;
+    should_clear_info_label = false;
   }
 
   // Add Wi-Fi networks.
@@ -719,6 +721,9 @@ NetworkListView::UpdateNetworkListEntries() {
   if (index == 0) {
     UpdateInfoLabel(IDS_ASH_STATUS_TRAY_NO_NETWORKS, index,
                     &no_wifi_networks_view_);
+  } else if (should_clear_info_label) {
+    // Update the label to show nothing.
+    UpdateInfoLabel(0, index, &no_wifi_networks_view_);
   }
 
   return new_guids;