ash: Clean up system background layer.

ash/ash_switches.cc

Index: ash/ash_switches.cc
diff --git a/ash/ash_switches.cc b/ash/ash_switches.cc
index 7ae43636ea42301e46a3483e97da3adc8693ddd1..836d5f799b5615ff7d2a2533b2850f073882142b 100644
--- a/ash/ash_switches.cc
+++ b/ash/ash_switches.cc
@@ -16,6 +16,12 @@ const char kAshBootAnimationFunction3[] = "ash-boot-animation-function3";
 // Constrains the pointer movement within a root window on desktop.
 const char kAshConstrainPointerToRoot[] = "ash-constrain-pointer-to-root";
 
+// Copies the host window's content to the system background layer at startup.
+// Can make boot slightly slower, but also hides an even-longer awkward period
+// where we display a white background if the login wallpaper takes a long time
+// to load.
+const char kAshCopyHostBackgroundAtBoot[] = "ash-copy-host-background-at-boot";
+
 // Enable keyboard shortcuts useful for debugging.
 const char kAshDebugShortcuts[] = "ash-debug-shortcuts";