mus: Makes ~WindowTreeClient reset context factory

ui/aura/mus/window_tree_client.h

Index: ui/aura/mus/window_tree_client.h
diff --git a/ui/aura/mus/window_tree_client.h b/ui/aura/mus/window_tree_client.h
index 535fcd3e5f8cb892f64dc02eb6e4175c04344365..3e42a2ef2ad82ef9eee1425bc0a0396e3f2e31d9 100644
--- a/ui/aura/mus/window_tree_client.h
+++ b/ui/aura/mus/window_tree_client.h
@@ -38,16 +38,14 @@ namespace gfx {
 class Insets;
 }
 
-namespace ui {
-struct PropertyData;
-}
-
 namespace service_manager {
 class Connector;
 }
 
 namespace ui {
+class ContextFactory;
 class Gpu;
+struct PropertyData;
 }
 
 namespace aura {
@@ -542,6 +540,10 @@ class AURA_EXPORT WindowTreeClient
 
   std::unique_ptr<ui::Gpu> gpu_;
   std::unique_ptr<MusContextFactory> compositor_context_factory_;
+
+  // If |compositor_context_factory_| is installed on Env, then this is the
+  // ContextFactory that was set on Env originally.
+  ui::ContextFactory* initial_context_factory_ = nullptr;
   base::WeakPtrFactory<WindowTreeClient> weak_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(WindowTreeClient);