[Fixed Position] Fix grid size spec

Fix spec which ensures that FixedController correctly
populates background grid based on user-defined grid
size, after changes for WTD-879.

platform/features/layout/src/FixedController.js

@@ -30,8 +30,12 @@ define(
             function refreshCellStyles() {
                 var x, y;
 
+                // Clear previous styles
                 cellStyles = [];
 
+                // Update grid size from model
+                gridSize = ($scope.model || {}).layoutGrid || gridSize;
+
                 for (x = 0; x < gridExtent[0]; x += 1) {
                     for (y = 0; y < gridExtent[1]; y += 1) {
                         // Position blocks; subtract out border size from w/h