GitLab

Add support for unique_ptr type of properties. Set their type converter
to be non-serializable.

Convert Combobox's 'model', 'owned_model', and 'callback' into
properties. And apply them into the example builder to show the usages.

Bug: 938501
Change-Id: I065f35420e266a0641754edf8edfebd8188f4144
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2477484
Commit-Queue: Wei Li <weili@chromium.org>
Reviewed-by: Peter Boström <pbos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#819383}