Make template type of data_grid/DataGrid.js more specific to prepare for migration to TS

Bug: chromium:1011811
Change-Id: I6395a1839cc22aebf410c5e6423f40d785dbd1c6
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/2476853
Commit-Queue: Andres Olivares <[email protected]>
Reviewed-by: Tim van der Lippe <[email protected]>
diff --git a/front_end/network/NetworkLogView.js b/front_end/network/NetworkLogView.js
index 9bc707c..4fe6f93 100644
--- a/front_end/network/NetworkLogView.js
+++ b/front_end/network/NetworkLogView.js
@@ -1124,7 +1124,9 @@
    * @return {!Array<!NetworkNode>}
    */
   flatNodesList() {
-    return /** @type {!Array<!NetworkNode>} */ (this._dataGrid.rootNode().flatChildren());
+    /** @type {!DataGrid.ViewportDataGrid.ViewportDataGridNode<!DataGrid.SortableDataGrid.SortableDataGridNode<!NetworkNode>>} */
+    const rootNode = (this._dataGrid.rootNode());
+    return /** @type {!Array<!NetworkNode>} */ (rootNode.flatChildren());
   }
 
   _onDataGridFocus() {