linux and chromeos: Turn on -Wunused-const-variable.

ash/wm/system_background_controller.cc

Index: ash/wm/system_background_controller.cc
diff --git a/ash/wm/system_background_controller.cc b/ash/wm/system_background_controller.cc
index 588073546decd94d83c2363bc1d6969ecef6eb5d..fd6e8ec15c46cc261a2af62d14c73dee0b5a42f9 100644
--- a/ash/wm/system_background_controller.cc
+++ b/ash/wm/system_background_controller.cc
@@ -11,15 +11,6 @@
 namespace ash {
 namespace internal {
 
-namespace {
-
-#if defined(OS_CHROMEOS)
-// Background color used for the Chrome OS boot splash screen.
-const SkColor kChromeOsBootColor = SkColorSetARGB(0xff, 0xfe, 0xfe, 0xfe);
-#endif
-
-}  // namespace
-
 SystemBackgroundController::SystemBackgroundController(
     aura::RootWindow* root_window,
     SkColor color)