DevTools: Don't allow excluding workspace roots

If a workspace root was exluded, it would instead exclude "undefined"

Change-Id: Ib82e11f2182a1fcced176735ae714212a153d5b2
Reviewed-on: https://chromium-review.googlesource.com/822457Reviewed-by: Andrey Lushnikov <lushnikov@chromium.org>
Commit-Queue: Andrey Lushnikov <lushnikov@chromium.org>
Cr-Commit-Position: refs/heads/master@{#523558}

third_party/WebKit/Source/devtools/front_end/sources/NavigatorView.js

@@ -724,8 +724,11 @@ Sources.NavigatorView = class extends UI.VBox {
 
     contextMenu.defaultSection().appendItem(
         Common.UIString('New file'), this._handleContextMenuCreate.bind(this, project, path));
-    contextMenu.defaultSection().appendItem(
-        Common.UIString('Exclude folder'), this._handleContextMenuExclude.bind(this, project, path));
+
+    if (!(node instanceof Sources.NavigatorGroupTreeNode)) {
+      contextMenu.defaultSection().appendItem(
+          Common.UIString('Exclude folder'), this._handleContextMenuExclude.bind(this, project, path));
+    }
 
     function removeFolder() {
       var shouldRemove = window.confirm(Common.UIString('Are you sure you want to remove this folder?'));