Show tooltip for UserAvatarButton.

The tooltip text for UserAvatarButon was set but not visible, because
can_process_event_within_subtree for its child view was true.

TEST=manual
BUG=853710

Change-Id: I081a96628b1ab77e4ca57ef7d5900777a864b297
Reviewed-on: https://chromium-review.googlesource.com/1107432Reviewed-by: Yoshiki Iguchi <yoshiki@chromium.org>
Commit-Queue: Tetsui Ohkubo <tetsui@chromium.org>
Cr-Commit-Position: refs/heads/master@{#569495}

ash/system/unified/user_chooser_view.cc

@@ -35,6 +35,7 @@ views::View* CreateUserAvatarView(int user_index) {
   DCHECK(user_session);
 
   auto* image_view = new tray::RoundedImageView(kTrayItemSize / 2);
+  image_view->set_can_process_events_within_subtree(false);
   if (user_session->user_info->type == user_manager::USER_TYPE_GUEST) {
     gfx::ImageSkia icon =
         gfx::CreateVectorIcon(kSystemMenuGuestIcon, kMenuIconColor);