Revert of Making WTH::InitCompositor() protected instead of public.

ash/mus/window_manager.cc

Index: ash/mus/window_manager.cc
diff --git a/ash/mus/window_manager.cc b/ash/mus/window_manager.cc
index c149054aa7e2389375bcffa56321761878d08e80..d65aad95cc2956f4483abc724410dff025e79d4c 100644
--- a/ash/mus/window_manager.cc
+++ b/ash/mus/window_manager.cc
@@ -212,6 +212,7 @@
 RootWindowController* WindowManager::CreateRootWindowController(
     std::unique_ptr<aura::WindowTreeHostMus> window_tree_host,
     const display::Display& display) {
+  window_tree_host->InitCompositor();
   // TODO(sky): this is temporary, should use RootWindowController directly.
   aura::client::SetCaptureClient(window_tree_host->window(),
                                  wm_state_->capture_controller());