Fix WindowTreeHostManager::GetPrimaryRootWindow.

ash/display/window_tree_host_manager.cc

Index: ash/display/window_tree_host_manager.cc
diff --git a/ash/display/window_tree_host_manager.cc b/ash/display/window_tree_host_manager.cc
index 358804ac0280a7b0ccecc20b30e24cb3c0b74b49..2408d979d385771ed4f43dce0be2b7841a2c2670 100644
--- a/ash/display/window_tree_host_manager.cc
+++ b/ash/display/window_tree_host_manager.cc
@@ -328,6 +328,10 @@ int64_t WindowTreeHostManager::GetPrimaryDisplayId() {
 }
 
 aura::Window* WindowTreeHostManager::GetPrimaryRootWindow() {
+  // If |primary_tree_host_for_replace_| is set, it means |primary_display_id|
+  // is kInvalidDisplayId.
+  if (primary_tree_host_for_replace_)
+    return GetWindow(primary_tree_host_for_replace_);
   return GetRootWindowForDisplayId(primary_display_id);
 }