[Blimp Engine] Adds a WindowTreeClient so that popup will have non-null paren…

blimp/engine/app/ui/blimp_window_tree_client.cc

+// 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 "blimp/engine/app/ui/blimp_window_tree_client.h"
+
+#include "ui/aura/window.h"
+
+namespace blimp {
+namespace engine {
+
+BlimpWindowTreeClient::BlimpWindowTreeClient(aura::Window* root_window)
+    : root_window_(root_window) {
+  aura::client::SetWindowTreeClient(root_window_, this);
+}
+
+BlimpWindowTreeClient::~BlimpWindowTreeClient() {
+  aura::client::SetWindowTreeClient(root_window_, nullptr);
+}
+
+aura::Window* BlimpWindowTreeClient::GetDefaultParent(aura::Window* context,
+                                                      aura::Window* window,
+                                                      const gfx::Rect& bounds) {
+  // Blimp has one root window. a null-parent window always attaches to it.

[Wez, 2016/03/30 00:07:55]

nit: Second sentence should start with a capital, or alternatively:

Suggest: "Always attach null-parented windows to the root window."

[haibinlu, 2016/03/30 01:40:07]

Done.

+  return root_window_;
+}
+
+}  // namespace engine
+}  // namespace blimp