mash: Migrate ScopedTargetRootWindow to //ash/common

ash/common/scoped_root_window_for_new_windows.cc

Index: ash/common/scoped_root_window_for_new_windows.cc
diff --git a/ash/common/scoped_root_window_for_new_windows.cc b/ash/common/scoped_root_window_for_new_windows.cc
new file mode 100644
index 0000000000000000000000000000000000000000..718b5d852177d8a6919d98c407165e0714885452
--- /dev/null
+++ b/ash/common/scoped_root_window_for_new_windows.cc
@@ -0,0 +1,22 @@
+// Copyright 2013 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/common/scoped_root_window_for_new_windows.h"
+
+#include "ash/common/wm_shell.h"
+#include "base/logging.h"
+
+namespace ash {
+
+ScopedRootWindowForNewWindows::ScopedRootWindowForNewWindows(WmWindow* new_root)
+    : old_root_(WmShell::Get()->root_window_for_new_windows()) {
+  DCHECK(new_root);
+  WmShell::Get()->set_root_window_for_new_windows(new_root);
+}
+
+ScopedRootWindowForNewWindows::~ScopedRootWindowForNewWindows() {
+  WmShell::Get()->set_root_window_for_new_windows(old_root_);

[msw, 2016/08/02 01:04:50]

I'm a little worried that this will break the value when multiple instances have
overlapping lifetimes, or when something calls set_root_window_for_new_windows()
directly during a ScopedRootWindowForNewWindows lifetime. Should WmShell use the
existing approach in ash::Shell? (keeping a separate member for scoped values,
setting that to null on scoped instance destruction, etc.) Otherwise, should
this [D]CHECK that the current value matches |new_root|?

[James Cook, 2016/08/02 16:16:04]

Good call. This was the source of my interactive_ui_tests failure on the bots.
Switched to the method used by ash::Shell.

+}
+
+}  // namespace ash