Show overlay displaying the state of all sticky keys when it is enabled.

ash/ash.gyp

Index: ash/ash.gyp
diff --git a/ash/ash.gyp b/ash/ash.gyp
index ecf4275a9d0283fe770776de7046c9eca2f2e480..787aa85357e9fe9d027179165d65a9b856f02cf0 100644
--- a/ash/ash.gyp
+++ b/ash/ash.gyp
@@ -250,8 +250,11 @@
         'shell_delegate.h',
         'shell_factory.h',
         'shell_window_ids.h',
+        'sticky_keys/sticky_keys_constants.h',
         'sticky_keys/sticky_keys_controller.cc',
         'sticky_keys/sticky_keys_controller.h',
+        'sticky_keys/sticky_keys_overlay.cc',
+        'sticky_keys/sticky_keys_overlay.h',
         'system/bluetooth/bluetooth_observer.h',
         'system/bluetooth/tray_bluetooth.cc',
         'system/bluetooth/tray_bluetooth.h',
@@ -870,6 +873,7 @@
         'shell/window_watcher_shelf_item_delegate.h',
         'shell/window_watcher_unittest.cc',
         'shell_unittest.cc',
+        'sticky_keys/sticky_keys_overlay_unittest.cc',
         'sticky_keys/sticky_keys_unittest.cc',
         'system/chromeos/managed/tray_locally_managed_user_unittest.cc',
         'system/chromeos/network/network_state_notifier_unittest.cc',
@@ -946,6 +950,7 @@
             ['exclude', 'ash_root_window_transformer_unittest.cc'],
             ['exclude', 'magnifier/magnification_controller_unittest.cc'],
             ['exclude', 'wm/workspace/workspace_window_resizer_unittest.cc'],
+            ['exclude', 'sticky_keys/sticky_keys_overlay_unittest.cc'],
             ['exclude', 'sticky_keys/sticky_keys_unittest.cc'],
             ['exclude', 'autoclick/autoclick_unittest.cc'],
           ],