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

blimp/engine/session/blimp_engine_session.h

Index: blimp/engine/session/blimp_engine_session.h
diff --git a/blimp/engine/session/blimp_engine_session.h b/blimp/engine/session/blimp_engine_session.h
index 1b311abe8fde1e2dbb9493f2ad69eb33693b1d7e..0e4aa654b5c2f0517edb717e126c6d78f36dda23 100644
--- a/blimp/engine/session/blimp_engine_session.h
+++ b/blimp/engine/session/blimp_engine_session.h
@@ -182,6 +182,9 @@ class BlimpEngineSession
   // Used to manage input capture.
   scoped_ptr<aura::client::DefaultCaptureClient> capture_client_;
 
+  // Used to attach null-parented windows (e.g. popups) to the root window.
+  scoped_ptr<aura::client::WindowTreeClient> window_tree_client_;
+
   // Only one web_contents is supported for blimp 0.5
   scoped_ptr<content::WebContents> web_contents_;