I got the solution actually i make a mistake in understanding your question sorry for inconvinience.
Now Solution for your problem: actually page_load event in code behind gets executed before javascript function so what you can do is use a session varibale like below.
In your page in which you want to make it post back
use below code
protected void Page_LoadComplete(object sender, EventArgs e)
{
if (Session["MakePostBack"] == null )
{
if (!IsPostBack)
FillGrid();
}
}
And in rest all pages where ever you are redirectiong to from this page
Session["MakePostBack"] = "PostItBack";
Hope This Helps!!!