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..b562c761315c556838ff12ffd4b825157d70b1b7 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);
 }
 
@@ -633,6 +637,8 @@ void WindowTreeHostManager::DeleteHost(AshWindowTreeHost* host_to_delete) {
   // Delete most of root window related objects, but don't delete
   // root window itself yet because the stack may be using it.
   controller->Shutdown();
+  if (primary_tree_host_for_replace_ == host_to_delete)
+    primary_tree_host_for_replace_ = nullptr;
   base::ThreadTaskRunnerHandle::Get()->DeleteSoon(FROM_HERE, controller);
 }