Mac a11y: Add Help, TopLevelUIElement, and Window accessibility attributes.

ui/views/accessibility/native_view_accessibility_auralinux.cc

Index: ui/views/accessibility/native_view_accessibility_auralinux.cc
diff --git a/ui/views/accessibility/native_view_accessibility_auralinux.cc b/ui/views/accessibility/native_view_accessibility_auralinux.cc
index 90927af368d2b9f10e44caa95c99ba607f86916e..cd54ecfc178c912c74912bcb84d49f1a301d9398 100644
--- a/ui/views/accessibility/native_view_accessibility_auralinux.cc
+++ b/ui/views/accessibility/native_view_accessibility_auralinux.cc
@@ -57,9 +57,7 @@ class AuraLinuxApplication
     return platform_node_->GetNativeViewAccessible();
   }
 
-  //
-  // WidgetObserver overrides.
-  //
+  // WidgetObserver:
 
   void OnWidgetDestroying(Widget* widget) override {
     auto iter = std::find(widgets_.begin(), widgets_.end(), widget);
@@ -67,14 +65,14 @@ class AuraLinuxApplication
       widgets_.erase(iter);
   }
 
-  //
-  // ui::AXPlatformNodeDelegate overrides.
-  //
+  // ui::AXPlatformNodeDelegate:
 
   const ui::AXNodeData& GetData() override {
     return data_;
   }
 
+  gfx::NativeWindow GetTopLevelWidget() override { return nullptr; }
+
   gfx::NativeViewAccessible GetParent() override {
     return nullptr;
   }