synchronize ash::kTabDraggingSourceWindowKey

This is a aura::Window pointer property, which can't be transported
as other properties. This CL allows registering such properties to
PropertyConverter, and WindowTreeClient/WindowTree takes care of
converting between the transport data and aura::Window pointer.

Bug: 867074
Test: manually
Change-Id: I3f3726264174b9291d2a779473ca5fae2d68266f
Reviewed-on: https://chromium-review.googlesource.com/1194789
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Jun Mukai <mukai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#588682}
18 files changed