Do not use assets for Ash window control button backgrounds

ash/frame/caption_buttons/frame_size_button_unittest.cc

Index: ash/frame/caption_buttons/frame_size_button_unittest.cc
diff --git a/ash/frame/caption_buttons/frame_size_button_unittest.cc b/ash/frame/caption_buttons/frame_size_button_unittest.cc
index 3c950c1f71ec53536b0fc075893be5c1489d1dce..59f6f6495d091996832ac21da6b87c1bcaf6a047 100644
--- a/ash/frame/caption_buttons/frame_size_button_unittest.cc
+++ b/ash/frame/caption_buttons/frame_size_button_unittest.cc
@@ -57,14 +57,12 @@ class TestWidgetDelegate : public views::WidgetDelegateView {
       caption_button_container_ =
           new FrameCaptionButtonContainerView(GetWidget());
 
-      // Set arbitrary images for the container's buttons so that the buttons
-      // have non-empty sizes.
+      // Set arbitrary images for the button icons and assign a non-empty size.
+      caption_button_container_->SetButtonSize(gfx::Size(32, 33));
       for (int icon = 0; icon < CAPTION_BUTTON_ICON_COUNT; ++icon) {
-        caption_button_container_->SetButtonImages(
+        caption_button_container_->SetButtonImage(
             static_cast<CaptionButtonIcon>(icon),
-            IDR_AURA_WINDOW_CONTROL_ICON_CLOSE,
-            IDR_AURA_WINDOW_CONTROL_BACKGROUND_H,
-            IDR_AURA_WINDOW_CONTROL_BACKGROUND_P);
+            IDR_AURA_WINDOW_CONTROL_ICON_CLOSE);
       }
 
       AddChildView(caption_button_container_);