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

blimp/engine/session/blimp_engine_session.cc

Index: blimp/engine/session/blimp_engine_session.cc
diff --git a/blimp/engine/session/blimp_engine_session.cc b/blimp/engine/session/blimp_engine_session.cc
index bd67782495d7aac59aabafe100ab5bf286210e10..0686e442bad33611cac682394921f3a524ecff44 100644
--- a/blimp/engine/session/blimp_engine_session.cc
+++ b/blimp/engine/session/blimp_engine_session.cc
@@ -13,6 +13,7 @@
 #include "blimp/engine/app/blimp_engine_config.h"
 #include "blimp/engine/app/ui/blimp_layout_manager.h"
 #include "blimp/engine/app/ui/blimp_screen.h"
+#include "blimp/engine/app/ui/blimp_window_tree_client.h"
 #include "blimp/engine/app/ui/blimp_window_tree_host.h"
 #include "blimp/engine/common/blimp_browser_context.h"
 #include "blimp/net/blimp_connection.h"
@@ -46,7 +47,6 @@
 #include "ui/wm/core/default_activation_client.h"
 #include "ui/wm/core/focus_controller.h"
 
-
 namespace blimp {
 namespace engine {
 namespace {
@@ -225,6 +225,9 @@ void BlimpEngineSession::Initialize() {
   capture_client_.reset(
       new aura::client::DefaultCaptureClient(window_tree_host_->window()));
 
+  window_tree_client_.reset(
+      new BlimpWindowTreeClient(window_tree_host_->window()));
+
   window_tree_host_->GetInputMethod()->AddObserver(this);
 
   window_tree_host_->SetBounds(gfx::Rect(screen_->GetPrimaryDisplay().size()));