You can code it to store the latlon into a hidden element. Example.
On the aspx page, add a hidden field
<asp:HiddenField ID="HiddenField1" runat="server" />
Then add the JavaScript
function showPosition(position) {
var latlon=position.coords.latitude+"||"+position.coords.longitude;
document.getElementById("<%=HiddenField1.ClientID%>").value = latlon;
}
Then in the code behind, access the hidden field value
var x = HiddenField1.Value;