Converts DockedWindowLayoutManager to common ash/wm types

ash/shell.cc

Index: ash/shell.cc
diff --git a/ash/shell.cc b/ash/shell.cc
index a24d502c0db8340311ae0b532fca1fc09cbc0b7c..fc7270b09a2d4795e6b1fa9833449e13378ae6b4 100644
--- a/ash/shell.cc
+++ b/ash/shell.cc
@@ -221,7 +221,6 @@ bool Shell::HasInstance() {
 // static
 void Shell::DeleteInstance() {
   delete instance_;
-  instance_ = nullptr;
 }
 
 // static
@@ -817,6 +816,10 @@ Shell::~Shell() {
   display_manager_->CreateScreenForShutdown();
   display_configuration_controller_.reset();
 
+  // Needs to happen before |window_tree_host_manager_|. Calls back to Shell, so
+  // also needs to be destroyed before |instance_| reset to null.
+  wm_globals_.reset();
+
   // Depends on |focus_client_|, so must be destroyed before.
   window_tree_host_manager_->Shutdown();
   window_tree_host_manager_.reset();