app_list: fix a bug.

When we show the launcher for the first time, we set the placeholder
text, but it doesn't show. This commit schedule to paint the placeholder
text when it changes.

Bug: 733662
Change-Id: I332c6e426171e19f4a035b6fa940f68f3721c058
Reviewed-on: https://chromium-review.googlesource.com/959436Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Jiaquan He <hejq@google.com>
Cr-Commit-Position: refs/heads/master@{#542803}

ui/app_list/views/search_box_view.cc

@@ -300,6 +300,7 @@ void SearchBoxView::HintTextChanged() {
       search_model_->search_box();
   search_box()->set_placeholder_text(search_box_model->hint_text());
   search_box()->SetAccessibleName(search_box_model->accessible_name());
+  SchedulePaint();
 }
 
 void SearchBoxView::SelectionModelChanged() {