Ensure compositor initialized before WindowTreeHost is Shown.

ui/aura/window_tree_host.cc

 // Copyright (c) 2013 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 "ui/aura/window_tree_host.h"
 
 #include "base/threading/thread_task_runner_handle.h"
 #include "base/trace_event/trace_event.h"
 #include "ui/aura/client/capture_client.h"
 #include "ui/aura/client/cursor_client.h"
@@ skipping 191 matching lines @@
   input_method_ = input_method;
   owned_input_method_ = false;
 }
 
 ui::EventDispatchDetails WindowTreeHost::DispatchKeyEventPostIME(
     ui::KeyEvent* event) {
   return SendEventToProcessor(event);
 }
 
 void WindowTreeHost::Show() {
-  if (compositor())
-    compositor()->SetVisible(true);
+  // Ensure that compositor has been properly initialized, see InitCompositor()
+  // and InitHost().
+  DCHECK(compositor());
+  DCHECK_EQ(compositor()->root_layer(), window()->layer());
+  compositor()->SetVisible(true);
   ShowImpl();
 }
 
 void WindowTreeHost::Hide() {
   HideImpl();
   if (compositor())
     compositor()->SetVisible(false);
 }
 
 ////////////////////////////////////////////////////////////////////////////////
@@ skipping 115 matching lines @@
   client::CursorClient* cursor_client = client::GetCursorClient(window());
   if (cursor_client) {
     const display::Display& display =
         display::Screen::GetScreen()->GetDisplayNearestWindow(window());
     cursor_client->SetDisplay(display);
   }
   dispatcher()->OnCursorMovedToRootLocation(root_location);
 }
 
 }  // namespace aura