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

blimp/engine/app/ui/blimp_window_tree_client.h

Index: blimp/engine/app/ui/blimp_window_tree_client.h
diff --git a/blimp/engine/app/ui/blimp_window_tree_client.h b/blimp/engine/app/ui/blimp_window_tree_client.h
new file mode 100644
index 0000000000000000000000000000000000000000..4fa57147ba581a7c947efbd6591dc998db79040e
--- /dev/null
+++ b/blimp/engine/app/ui/blimp_window_tree_client.h
@@ -0,0 +1,35 @@
+// 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.
+
+#ifndef BLIMP_ENGINE_APP_UI_BLIMP_WINDOW_TREE_CLIENT_H_
+#define BLIMP_ENGINE_APP_UI_BLIMP_WINDOW_TREE_CLIENT_H_
+
+#include "base/macros.h"
+#include "ui/aura/client/window_tree_client.h"
+
+namespace blimp {
+namespace engine {
+
+class BlimpWindowTreeClient : public aura::client::WindowTreeClient {
+ public:
+  // Caller ensures that |root_window| outlives this object.
+  explicit BlimpWindowTreeClient(aura::Window* root_window);
+
+  ~BlimpWindowTreeClient() override;
+
+  // aura::client::WindowTreeClient interface.
+  aura::Window* GetDefaultParent(aura::Window* context,
+                                 aura::Window* window,
+                                 const gfx::Rect& bounds) override;
+
+ private:
+  aura::Window* root_window_;
+
+  DISALLOW_COPY_AND_ASSIGN(BlimpWindowTreeClient);
+};
+
+}  // namespace engine
+}  // namespace blimp
+
+#endif  // BLIMP_ENGINE_APP_UI_BLIMP_WINDOW_TREE_CLIENT_H_