Converts ash to use aura-mus

ash/mus/screen_mus.cc

Index: ash/mus/screen_mus.cc
diff --git a/ash/mus/screen_mus.cc b/ash/mus/screen_mus.cc
new file mode 100644
index 0000000000000000000000000000000000000000..312eb9d6505ed29473565263396008d7b88e35f9
--- /dev/null
+++ b/ash/mus/screen_mus.cc
@@ -0,0 +1,26 @@
+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "ash/mus/screen_mus.h"
+
+#include "ui/aura/mus/window_tree_host_mus.h"
+#include "ui/aura/window.h"
+
+namespace ash {
+
+ScreenMus::ScreenMus() = default;
+
+ScreenMus::~ScreenMus() = default;
+
+display::Display ScreenMus::GetDisplayNearestWindow(
+    aura::Window* window) const {
+  const aura::WindowTreeHost* host = window->GetHost();
+  if (!host)
+    return GetPrimaryDisplay();
+  auto iter = display_list().FindDisplayById(
+      static_cast<const aura::WindowTreeHostMus*>(host)->display_id());
+  return iter == display_list().displays().end() ? GetPrimaryDisplay() : *iter;
+}
+
+}  // namespace ash