[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_, NULL);

[Wez, 2016/03/29 21:20:06]

NULL->nullptr?

[haibinlu, 2016/03/29 21:57:15]

Done.

+}
+
+aura::Window* BlimpWindowTreeClient::GetDefaultParent(aura::Window* context,
+                                                      aura::Window* window,
+                                                      const gfx::Rect& bounds) {
+  return root_window_;

[Wez, 2016/03/29 21:20:06]

Add a brief comment explaining why we use |root_window_| and ignore |context|
and |window| - should we be checking anything about the parameters?

[haibinlu, 2016/03/29 21:57:15]

Added comment. 

Per window_tree_client comments, context and bounds are planed to be removed. //
TODO(erg): Remove |context|, and maybe after oshima's patch lands, |bounds|.

+}
+
+}  // namespace engine
+}  // namespace blimp