[WIP] ash/extensions: Add experimental extension support for window-management in ash.

ash/shell_delegate.h

Index: ash/shell_delegate.h
diff --git a/ash/shell_delegate.h b/ash/shell_delegate.h
index 8029e1b9686ac0581fd26bf3e987d3b88a893a50..56e8c078b47868cbdc1ba581daf74851b7e1d383 100644
--- a/ash/shell_delegate.h
+++ b/ash/shell_delegate.h
@@ -35,6 +35,7 @@ struct LauncherItem;
 class SystemTray;
 class SystemTrayDelegate;
 class UserWallpaperDelegate;
+class WindowManagerExtensionDelegate;
 
 enum UserMetricsAction {
   UMA_ACCEL_NEWTAB_T,
@@ -132,6 +133,9 @@ class ASH_EXPORT ShellDelegate {
   // Creates a user action client. Shell takes ownership of the object.
   virtual aura::client::UserActionClient* CreateUserActionClient() = 0;
 
+  virtual ash::WindowManagerExtensionDelegate*
+      CreateWindowManagerDelegate() = 0;
+
   // Opens the feedback page for "Report Issue".
   virtual void OpenFeedbackPage() = 0;