assistant: Add a11y heading to query log

This cl adds a11y heading to query log in the AssistantQueryView.

Bug: b/113734499
Test: tested on EVE
Change-Id: I5dcca50ef2b6a28e050b3dc462437c60036c56d7
Reviewed-on: https://chromium-review.googlesource.com/c/1285051
Commit-Queue: Tao Wu <wutao@chromium.org>
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#600932}

ash/assistant/ui/main_stage/assistant_main_stage.cc

@@ -502,6 +502,12 @@ bool AssistantMainStage::OnActiveQueryExitAnimationEnded(
     const ui::CallbackLayerAnimationObserver& observer) {
   // The exited active query view will always be the first child of its parent.
   delete query_layout_container_->child_at(0);
+
+  // TODO(https://crbug.com/896079): Remove this when view.cc handles the
+  // event notification.
+  query_layout_container_->NotifyAccessibilityEvent(
+      ax::mojom::Event::kChildrenChanged, false);
+
   UpdateTopPadding();
 
   // Return false to prevent the observer from destroying itself.

ash/assistant/ui/main_stage/assistant_query_view.cc

@@ -10,6 +10,8 @@
 #include "ash/assistant/model/assistant_query.h"
 #include "ash/assistant/ui/assistant_ui_constants.h"
 #include "base/strings/utf_string_conversions.h"
+#include "ui/accessibility/ax_enums.mojom.h"
+#include "ui/views/accessibility/view_accessibility.h"
 #include "ui/views/layout/box_layout.h"
 
 namespace ash {
@@ -37,6 +39,7 @@ views::StyledLabel::RangeStyleInfo CreateStyleInfo(SkColor color) {
 
 AssistantQueryView::AssistantQueryView() {
   InitLayout();
+  GetViewAccessibility().OverrideRole(ax::mojom::Role::kHeading);
 }
 
 AssistantQueryView::~AssistantQueryView() = default;