Fix passing host name property through shill Manager interface.

https://android-review.googlesource.com/c/platform/system/connectivity/shill/+/181040
replaced the "HostName" property in the shill Manager interface with a new
"DHCPProperty.Hostname" property. However, the migration was partially done,
which has been fixed by CL:1102152. This CL updates the code on the Chrome side
accordingly.

Bug: 852035
Change-Id: Ib0d7fc7e82f13b9f77cda13324702f610d952f61
Reviewed-on: https://chromium-review.googlesource.com/1104312Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Commit-Queue: Ben Chan <benchan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#568092}

chromeos/network/shill_property_handler.cc

@@ -233,7 +233,7 @@ void ShillPropertyHandler::SetWakeOnLanEnabled(bool enabled) {
 void ShillPropertyHandler::SetHostname(const std::string& hostname) {
   base::Value value(hostname);
   shill_manager_->SetProperty(
-      shill::kHostNameProperty, value, base::DoNothing(),
+      shill::kDhcpPropertyHostnameProperty, value, base::DoNothing(),
       base::BindRepeating(&network_handler::ShillErrorCallbackFunction,
                           "SetHostname Failed", "Manager",
                           network_handler::ErrorCallback()));