Converts DockedWindowLayoutManager to use WmRootWindowControllerObserver

ash/wm/aura/wm_window_aura.cc

Index: ash/wm/aura/wm_window_aura.cc
diff --git a/ash/wm/aura/wm_window_aura.cc b/ash/wm/aura/wm_window_aura.cc
index 9bbde25f21be0d6220b47363edba6e25bb10bf01..8e73ea8cb0fa877e97688745acf5cdeed732a0e7 100644
--- a/ash/wm/aura/wm_window_aura.cc
+++ b/ash/wm/aura/wm_window_aura.cc
@@ -361,6 +361,10 @@ void WmWindowAura::SetRestoreShowState(ui::WindowShowState show_state) {
   window_->SetProperty(aura::client::kRestoreShowStateKey, show_state);
 }
 
+void WmWindowAura::SetLockedToRoot(bool value) {
+  window_->SetProperty(kStayInSameRootWindowKey, value);
+}
+
 void WmWindowAura::SetCapture() {
   window_->SetCapture();
 }