Is it reasonable to convert these to string window properties, instead of members?

As they aren't used on all windows, sure.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/5f2fe79fcb79b28111dcdb37e226cff0a1ee5745

commit 5f2fe79fcb79b28111dcdb37e226cff0a1ee5745
Author: msw <msw@chromium.org>
Date: Fri Nov 18 06:14:32 2016

Use aura::WindowProperty for title and name strings.

Makes it easier to convert/transport these for mus.

BUG= 665245 
TEST=No window titling/naming regressions.
R=sky@chromium.org

Review-Url: https://codereview.chromium.org/2502413005
Cr-Commit-Position: refs/heads/master@{#433130}

[modify] https://crrev.com/5f2fe79fcb79b28111dcdb37e226cff0a1ee5745/ash/ash_touch_exploration_manager_chromeos.cc
[modify] https://crrev.com/5f2fe79fcb79b28111dcdb37e226cff0a1ee5745/ash/aura/wm_window_aura.cc
[modify] https://crrev.com/5f2fe79fcb79b28111dcdb37e226cff0a1ee5745/ash/display/root_window_transformers_unittest.cc
[modify] https://crrev.com/5f2fe79fcb79b28111dcdb37e226cff0a1ee5745/ash/display/window_tree_host_manager_unittest.cc
[modify] https://crrev.com/5f2fe79fcb79b28111dcdb37e226cff0a1ee5745/ash/shell/window_watcher_shelf_item_delegate.cc
[modify] https://crrev.com/5f2fe79fcb79b28111dcdb37e226cff0a1ee5745/ash/touch/touch_uma.cc
[modify] https://crrev.com/5f2fe79fcb79b28111dcdb37e226cff0a1ee5745/ash/wm/window_util.cc
[modify] https://crrev.com/5f2fe79fcb79b28111dcdb37e226cff0a1ee5745/ash/wm/workspace_controller_unittest.cc
[modify] https://crrev.com/5f2fe79fcb79b28111dcdb37e226cff0a1ee5745/chrome/browser/media/webrtc/desktop_media_list_ash.cc
[modify] https://crrev.com/5f2fe79fcb79b28111dcdb37e226cff0a1ee5745/chrome/browser/memory/tab_manager_delegate_chromeos.cc
[modify] https://crrev.com/5f2fe79fcb79b28111dcdb37e226cff0a1ee5745/chrome/browser/ui/ash/launcher/arc_app_window_launcher_item_controller.cc
[modify] https://crrev.com/5f2fe79fcb79b28111dcdb37e226cff0a1ee5745/ui/aura/client/aura_constants.cc
[modify] https://crrev.com/5f2fe79fcb79b28111dcdb37e226cff0a1ee5745/ui/aura/client/aura_constants.h
[modify] https://crrev.com/5f2fe79fcb79b28111dcdb37e226cff0a1ee5745/ui/aura/window.cc
[modify] https://crrev.com/5f2fe79fcb79b28111dcdb37e226cff0a1ee5745/ui/aura/window.h
[modify] https://crrev.com/5f2fe79fcb79b28111dcdb37e226cff0a1ee5745/ui/views/accessibility/ax_window_obj_wrapper.cc
[modify] https://crrev.com/5f2fe79fcb79b28111dcdb37e226cff0a1ee5745/ui/views/mus/desktop_window_tree_host_mus.cc
[modify] https://crrev.com/5f2fe79fcb79b28111dcdb37e226cff0a1ee5745/ui/views/widget/native_widget_aura.cc
[modify] https://crrev.com/5f2fe79fcb79b28111dcdb37e226cff0a1ee5745/ui/views/widget/window_reorderer_unittest.cc
[modify] https://crrev.com/5f2fe79fcb79b28111dcdb37e226cff0a1ee5745/ui/wm/public/tooltip_client.cc

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/c996ed6fe7db9b512d465ae65dc85d30b7efc745

commit c996ed6fe7db9b512d465ae65dc85d30b7efc745
Author: msw <msw@chromium.org>
Date: Sat Nov 19 23:55:21 2016

Add string16 and name/title support to aura::PropertyConverter.

BUG= 665245 
TEST=Automated
R=sky@chromium.org

Review-Url: https://codereview.chromium.org/2513753003
Cr-Commit-Position: refs/heads/master@{#433428}

[modify] https://crrev.com/c996ed6fe7db9b512d465ae65dc85d30b7efc745/ui/aura/mus/property_converter.cc
[modify] https://crrev.com/c996ed6fe7db9b512d465ae65dc85d30b7efc745/ui/aura/mus/property_converter.h
[modify] https://crrev.com/c996ed6fe7db9b512d465ae65dc85d30b7efc745/ui/aura/mus/property_converter_unittest.cc
[modify] https://crrev.com/c996ed6fe7db9b512d465ae65dc85d30b7efc745/ui/aura/mus/window_tree_client_unittest.cc

This should be fixed; let me know if anything is missing.