Centralizes more shared code between ash and mash

ash/mus/bridge/wm_window_mus.cc

Index: ash/mus/bridge/wm_window_mus.cc
diff --git a/ash/mus/bridge/wm_window_mus.cc b/ash/mus/bridge/wm_window_mus.cc
index d3fc487382eb1c649522c705c776f28c6317c001..d0234c695586fd97b05db357c1b954d07c1212b0 100644
--- a/ash/mus/bridge/wm_window_mus.cc
+++ b/ash/mus/bridge/wm_window_mus.cc
@@ -161,6 +161,8 @@ bool WmWindowMus::IsContainer() const {
 }
 
 void WmWindowMus::Destroy() {
+  // TODO(sky): to match aura behavior this should delete children.
+  // http://crbug.com/647513.
   window_->Destroy();
   // WARNING: this has been deleted.
 }
@@ -444,6 +446,10 @@ void WmWindowMus::AddChild(WmWindow* window) {
   window_->AddChild(GetMusWindow(window));
 }
 
+void WmWindowMus::RemoveChild(WmWindow* child) {
+  window_->RemoveChild(GetMusWindow(child));
+}
+
 const WmWindow* WmWindowMus::GetParent() const {
   return Get(window_->parent());
 }