ozone: xkb: Load keymaps on worker thread & cache them

ui/events/ozone/layout/xkb/scoped_xkb.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef UI_EVENTS_OZONE_LAYOUT_XKB_SCOPED_XKB_H_
 #define UI_EVENTS_OZONE_LAYOUT_XKB_SCOPED_XKB_H_
 
 #include <xkbcommon/xkbcommon.h>
 
 namespace ui {
 
 // libxkbcommon uses explicit reference counting for its structures,
 // so we need to trigger its cleanup.
 struct XkbContextDeleter {
   void operator()(xkb_context* context) { xkb_context_unref(context); }
 };
 
 struct XkbStateDeleter {
   void operator()(xkb_state* state) { xkb_state_unref(state); }
 };
 
+struct XkbKeymapDeleter {
+  void operator()(xkb_keymap* keymap) { xkb_keymap_unref(keymap); }
+};
+
 }  // namespace ui
 
 #endif  // UI_EVENTS_OZONE_LAYOUT_XKB_SCOPED_XKB_H_