Ensure compositor initialized before WindowTreeHost is Shown.

ui/aura/window_tree_host.cc

Index: ui/aura/window_tree_host.cc
diff --git a/ui/aura/window_tree_host.cc b/ui/aura/window_tree_host.cc
index 87fac84f1cfdfdd5241b829cdd23697b119055f6..4c5cd49161d0473f7c7cd3a8f038d006b6a901d1 100644
--- a/ui/aura/window_tree_host.cc
+++ b/ui/aura/window_tree_host.cc
@@ -209,8 +209,11 @@ ui::EventDispatchDetails WindowTreeHost::DispatchKeyEventPostIME(
 }
 
 void WindowTreeHost::Show() {
-  if (compositor())
-    compositor()->SetVisible(true);
+  // Ensure that compositor has been properly initialized, see InitCompositor()
+  // and InitHost().
+  CHECK(compositor());

[sadrul, 2016/12/06 16:10:54]

DCHECK() should be fine here too.

+  DCHECK_EQ(compositor()->root_layer(), window()->layer());
+  compositor()->SetVisible(true);
   ShowImpl();
 }