Add tooltip to close button in ChromeOS search box

Use the same text as for a11y name.

Bug: 794441
Test: manual
Change-Id: Ifdbf19b20d2ded8071dd8187083af832c1357f22
Reviewed-on: https://chromium-review.googlesource.com/c/1460074
Commit-Queue: Vladislav Kaznacheev <kaznacheev@chromium.org>
Commit-Queue: Alex Newcomer <newcomer@chromium.org>
Auto-Submit: Vladislav Kaznacheev <kaznacheev@chromium.org>
Reviewed-by: Alex Newcomer <newcomer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#630137}

ash/app_list/views/search_box_view.cc

@@ -204,8 +204,10 @@ void SearchBoxView::SetupCloseButton() {
                   gfx::CreateVectorIcon(views::kIcCloseIcon, kCloseIconSize,
                                         search_box_color()));
   close->SetVisible(false);
-  close->SetAccessibleName(
+  base::string16 close_button_label(
       l10n_util::GetStringUTF16(IDS_APP_LIST_CLEAR_SEARCHBOX));
+  close->SetAccessibleName(close_button_label);
+  close->SetTooltipText(close_button_label);
 }
 
 void SearchBoxView::SetupBackButton() {