Add a local PropertyTrees copy to ui::Compositor.

As part of migrating ui::Compositor to using cc::Compositor in
layer list / property_tree mode, I'm going to maintain a parallel
property_tree in the ui::Compositor that will gradually get closer
and closer to the one the cc::Compositor actually uses, with the
goal being eventually to switch to using the ui-maintained one
directly.

This CL adds an empty PropertyTree to the ui::Compositor and a
hook for checking its correctness, which right now does nothing.

This CL should have no functional effect.

Bug: 389771428
Change-Id: Ic7c28482fb08ce8678d5b917d73af1d1ad71c3d7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6237374
Reviewed-by: Jonathan Ross <jonross@chromium.org>
Commit-Queue: Dirk Pranke <dpranke@google.com>
Cr-Commit-Position: refs/heads/main@{#1421477}