Fix WindowTreeHostManager::GetPrimaryRootWindow.

ash/shell.cc

Index: ash/shell.cc
diff --git a/ash/shell.cc b/ash/shell.cc
index 525229c8e6cc076d334e02aebc2ca5f31855576e..d12c07fe48a4c8c898637e1278223e7d81d327e1 100644
--- a/ash/shell.cc
+++ b/ash/shell.cc
@@ -269,7 +269,8 @@ RootWindowController* Shell::GetRootWindowControllerWithDisplayId(
 // static
 aura::Window* Shell::GetPrimaryRootWindow() {
   CHECK(HasInstance());
-  return instance_->shell_port_->GetPrimaryRootWindow()->aura_window();
+  WmWindow* wm_window = instance_->shell_port_->GetPrimaryRootWindow();

[varkha, 2017/05/10 22:23:59]

Not for this CL - WmWindow should probably go away here as we are using Aura as
a client lib in mus_ash. Maybe a TODO to make shell_port.h use aura::Window* in
place of WmWindow *.

+  return wm_window ? wm_window->aura_window() : nullptr;
 }
 
 // static