Fix font rendering in experimental app list tiles.

This CL fixes font rendering for experimental app list tiles by setting
a background.

BUG=406989

Review URL: https://codereview.chromium.org/499983002

Cr-Commit-Position: refs/heads/master@{#291849}

ui/app_list/views/tile_item_view.cc

@@ -14,6 +14,7 @@
 #include "ui/gfx/canvas.h"
 #include "ui/gfx/color_analysis.h"
 #include "ui/gfx/color_utils.h"
+#include "ui/views/background.h"
 #include "ui/views/controls/image_view.h"
 #include "ui/views/controls/label.h"
 #include "ui/views/layout/box_layout.h"
@@ -43,6 +44,8 @@ TileItemView::TileItemView()
   ui::ResourceBundle& rb = ui::ResourceBundle::GetSharedInstance();
   title_->SetAutoColorReadabilityEnabled(false);
   title_->SetEnabledColor(kGridTitleColor);
+  title_->set_background(views::Background::CreateSolidBackground(
+      app_list::kContentsBackgroundColor));
   title_->SetFontList(rb.GetFontList(kItemTextFontStyle));
   title_->SetHorizontalAlignment(gfx::ALIGN_CENTER);