Implement TOGGLE_EXPANSION action for the ARC notifications.

ui/arc/notification/arc_custom_notification_view.cc

Index: ui/arc/notification/arc_custom_notification_view.cc
diff --git a/ui/arc/notification/arc_custom_notification_view.cc b/ui/arc/notification/arc_custom_notification_view.cc
index 908e49eff5660a854f1c04ccbb43df91d67db0c6..3e6a9818f1ce6da4bda8c4c0001aa38e7c1fcece 100644
--- a/ui/arc/notification/arc_custom_notification_view.cc
+++ b/ui/arc/notification/arc_custom_notification_view.cc
@@ -547,6 +547,22 @@ views::FocusTraversable* ArcCustomNotificationView::GetFocusTraversable() {
   return nullptr;
 }
 
+bool ArcCustomNotificationView::OnMousePressed(const ui::MouseEvent& event) {
+  // TODO(yhanada): Remove this hack as soon as possible after letting
+  // accessible actions be delivered to this view.
+  // All mouse clicks or touches should be sent to corresponding Android view
+  // because the surface is on this view, so receiving a mouse pressed event
+  // means the event is generated by automation API.
+  if (event.IsOnlyLeftMouseButton() &&
+      item_->expand_state() !=
+          message_center::NotificationExpandState::FIXED_SIZE &&
+      event.target() != surface_->window()) {
+    item_->ToggleExpansion();
+    return true;
+  }
+  return false;
+}
+
 void ArcCustomNotificationView::ButtonPressed(views::Button* sender,
                                               const ui::Event& event) {
   if (item_ && !item_->pinned() && sender == close_button_.get()) {