[chromecast] Prevent ax::Role List with size zero.

Make sure a set the semantic's role as kList only when it has actionable
children. Otherwise it is just a GenericContainer.

Bug: b/178063833
Test: test on assistant device.
Change-Id: Ie309e7bd3b87907fea19dbd1896f886965c7752a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2643478Reviewed-by: Randy Rossi <rmrossi@chromium.org>
Commit-Queue: Yuanyao Zhong <yyzhong@google.com>
Cr-Commit-Position: refs/heads/master@{#845785}

chromecast/browser/accessibility/flutter/flutter_semantics_node_wrapper.cc

@@ -171,7 +171,9 @@ void FlutterSemanticsNodeWrapper::PopulateAXRole(
     return;
   }
 
-  if (node_ptr_->scroll_children() > 0) {
+  std::vector<FlutterSemanticsNodeWrapper*> actionable_children;
+  GetActionableChildren(&actionable_children);
+  if (node_ptr_->scroll_children() > 0 && actionable_children.size() > 0) {
     out_data->role = ax::mojom::Role::kList;
     return;
   }