color_utils cleanup:

apps/ui/views/app_window_frame_view.cc

Index: apps/ui/views/app_window_frame_view.cc
diff --git a/apps/ui/views/app_window_frame_view.cc b/apps/ui/views/app_window_frame_view.cc
index 4b6246b2b8ded9cd8cb2820b581f72bbb30328c6..e963fdd64690fdb95950ee5f9acdd4a8bd24564a 100644
--- a/apps/ui/views/app_window_frame_view.cc
+++ b/apps/ui/views/app_window_frame_view.cc
@@ -374,10 +374,8 @@ void AppWindowFrameView::SetButtonImagesForFrame() {
 
   // If the frame is dark, we should use the light images so they have
   // some contrast.
-  unsigned char frame_luma =
-      color_utils::GetLuminanceForColor(CurrentFrameColor());
-  const unsigned char kLuminanceThreshold = 100;
-  bool use_light = frame_luma < kLuminanceThreshold;
+  const uint8_t kLumaThreshold = 100;
+  bool use_light = color_utils::Luma(CurrentFrameColor()) < kLumaThreshold;
 
   ui::ResourceBundle& rb = ui::ResourceBundle::GetSharedInstance();
   if (use_light) {