ui + mus: Split ContextFactory into ContextFactory(Client) and ContextFactoryPrivate

ui/compositor/test/in_process_context_factory.h

Index: ui/compositor/test/in_process_context_factory.h
diff --git a/ui/compositor/test/in_process_context_factory.h b/ui/compositor/test/in_process_context_factory.h
index 68914332747fe23222ec39e1e6d556731b735f96..8d91dc21493e8bc4d5386307e167e6b70674975e 100644
--- a/ui/compositor/test/in_process_context_factory.h
+++ b/ui/compositor/test/in_process_context_factory.h
@@ -24,7 +24,8 @@ class SurfaceManager;
 namespace ui {
 class InProcessContextProvider;
 
-class InProcessContextFactory : public ContextFactory {
+class InProcessContextFactory : public ContextFactory,
+                                public ContextFactoryPrivate {
  public:
   // surface_manager is owned by the creator of this and must outlive the
   // context factory.