Centralizes more shared code between ash and mash

ash/aura/wm_window_aura.cc

Index: ash/aura/wm_window_aura.cc
diff --git a/ash/aura/wm_window_aura.cc b/ash/aura/wm_window_aura.cc
index 60575f4a34e474bab4421d5c5c4ad20ce00bf6ee..831751f6d9d2f585bdbdcf352c558824390b0c71 100644
--- a/ash/aura/wm_window_aura.cc
+++ b/ash/aura/wm_window_aura.cc
@@ -375,6 +375,10 @@ void WmWindowAura::AddChild(WmWindow* window) {
   window_->AddChild(GetAuraWindow(window));
 }
 
+void WmWindowAura::RemoveChild(WmWindow* child) {
+  window_->RemoveChild(GetAuraWindow(child));
+}
+
 const WmWindow* WmWindowAura::GetParent() const {
   return Get(window_->parent());
 }