Refactor backdrop that is currently used in the maximized mode.

components/exo/wm_helper_mus.cc

Index: components/exo/wm_helper_mus.cc
diff --git a/components/exo/wm_helper_mus.cc b/components/exo/wm_helper_mus.cc
index a13d09ff997059e1edfb87b5da58f06121da22e5..105691c8c44cfd5e09e00ceeb087e1ddcc0fdbf2 100644
--- a/components/exo/wm_helper_mus.cc
+++ b/components/exo/wm_helper_mus.cc
@@ -86,15 +86,6 @@ bool WMHelperMus::IsMaximizeModeWindowManagerEnabled() const {
   return false;
 }
 
-bool WMHelperMus::IsSpokenFeedbackEnabled() const {
-  NOTIMPLEMENTED();
-  return false;
-}
-
-void WMHelperMus::PlayEarcon(int sound_key) const {
-  NOTIMPLEMENTED();
-}
-
 void WMHelperMus::OnActiveFocusClientChanged(
     aura::client::FocusClient* focus_client,
     aura::Window* focus_client_root) {