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

blimp/engine/app/ui/blimp_window_tree_client.cc

Index: blimp/engine/app/ui/blimp_window_tree_client.cc
diff --git a/blimp/engine/app/ui/blimp_window_tree_client.cc b/blimp/engine/app/ui/blimp_window_tree_client.cc
new file mode 100644
index 0000000000000000000000000000000000000000..6524ebb0f5227a67b8b6dda365fb04bd7a5359df
--- /dev/null
+++ b/blimp/engine/app/ui/blimp_window_tree_client.cc
@@ -0,0 +1,30 @@
+// 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. Always attach null-parented windows to the root
+  // window.
+  return root_window_;
+}
+
+}  // namespace engine
+}  // namespace blimp