Fix on-screen keyboard issue when search box is focused

Background:
In some devices like nocturne, on-screen keyboard pops up when launcher
is focused. When launcher is focused, search box textfield will be the
initial focused view. SearchBoxViewBase::ContentsChanged() gets called
to activate the search box, which triggers the on-screen keyboard.

Change:
Avoid activate the search box when new contents are empty string.

Bug: 895249
Change-Id: I32b0f5a76037b5287d1b179fa20140101ce4f853
Reviewed-on: https://chromium-review.googlesource.com/c/1284754Reviewed-by: Alex Newcomer <newcomer@chromium.org>
Commit-Queue: Weidong Guo <weidongg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#600577}

ui/chromeos/search_box/search_box_view_base.cc

@@ -490,8 +490,8 @@ void SearchBoxViewBase::UpdateButtonsVisisbility() {
   if (close_button_->visible() == should_show_close_button &&
       assistant_button_->visible() == should_show_assistant_button &&
       search_box_right_space_->visible() ==
-      should_show_search_box_right_space) {
-      return;
+          should_show_search_box_right_space) {
+    return;
   }
 
   close_button_->SetVisible(should_show_close_button);
@@ -507,7 +507,8 @@ void SearchBoxViewBase::ContentsChanged(views::Textfield* sender,
   search_box_->RequestFocus();
   UpdateModel(true);
   NotifyQueryChanged();
-  SetSearchBoxActive(true, ui::ET_KEY_PRESSED);
+  if (!new_contents.empty())
+    SetSearchBoxActive(true, ui::ET_KEY_PRESSED);
   UpdateButtonsVisisbility();
 }