Adds some TODOs with references to bugs

ui/views/mus/desktop_window_tree_host_mus.cc

Index: ui/views/mus/desktop_window_tree_host_mus.cc
diff --git a/ui/views/mus/desktop_window_tree_host_mus.cc b/ui/views/mus/desktop_window_tree_host_mus.cc
index b1f420f344dc53e3e94a77cf10dd172355180800..f42e00a7c8c9789df77403dc519147a61218656e 100644
--- a/ui/views/mus/desktop_window_tree_host_mus.cc
+++ b/ui/views/mus/desktop_window_tree_host_mus.cc
@@ -131,10 +131,12 @@ void DesktopWindowTreeHostMus::SetSize(const gfx::Size& size) {
 }
 
 void DesktopWindowTreeHostMus::StackAbove(aura::Window* window) {
+  // TODO: implement window stacking, http://crbug.com/663617.
   NOTIMPLEMENTED();
 }
 
 void DesktopWindowTreeHostMus::StackAtTop() {
+  // TODO: implement window stacking, http://crbug.com/663617.
   NOTIMPLEMENTED();
 }
 
@@ -218,6 +220,7 @@ void DesktopWindowTreeHostMus::Activate() {
 
 void DesktopWindowTreeHostMus::Deactivate() {
   // TODO: Deactivate() means focus next window, that needs to go to mus.
+  // http://crbug.com/663618.
   NOTIMPLEMENTED();
 }
 
@@ -328,7 +331,7 @@ bool DesktopWindowTreeHostMus::IsFullscreen() const {
 }
 void DesktopWindowTreeHostMus::SetOpacity(float opacity) {
   // TODO: this likely need to go to server so that non-client decorations get
-  // opacity.
+  // opacity. http://crbug.com/663619.
   window()->layer()->SetOpacity(opacity);
 }