assistant: notify screen reader to read query result

Bug: 113734839
Test: "tell me a joke" reads the joke out through screen reader
Change-Id: I92832ff6e7eea298b59aec660763b0d1a4854eef
Reviewed-on: https://chromium-review.googlesource.com/1217751
Commit-Queue: Xiaohui Chen <xiaohuic@chromium.org>
Reviewed-by: Muyuan Li <muyuanli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#590541}

ash/assistant/ui/main_stage/ui_element_container_view.cc

@@ -4,6 +4,8 @@
 
 #include "ash/assistant/ui/main_stage/ui_element_container_view.h"
 
+#include <string>
+
 #include "ash/assistant/assistant_controller.h"
 #include "ash/assistant/assistant_interaction_controller.h"
 #include "ash/assistant/model/assistant_response.h"
@@ -363,6 +365,11 @@ void UiElementContainerView::OnAllUiElementsAdded() {
                 kUiElementAnimationFadeInDelay),
             CreateOpacityElement(1.f, kUiElementAnimationFadeInDuration)));
   }
+
+  // Let screen reader read the query result.
+  // NOTE: this won't read webview result, which will be triggered with HTML
+  // ARIA.
+  NotifyAccessibilityEvent(ax::mojom::Event::kAlert, true);
 }
 
 bool UiElementContainerView::OnAllUiElementsExitAnimationEnded(

ash/assistant/ui/main_stage/ui_element_container_view.h

@@ -7,6 +7,7 @@
 
 #include <deque>
 #include <memory>
+#include <utility>
 #include <vector>
 
 #include "ash/assistant/model/assistant_interaction_model_observer.h"