This patch moves initialization of inobject_properties and
unused_property_fields of a map to the construction time of the map.

Map::AppendDescriptor now properly decrements unused_property_fields
and thus maintains the invariant for property field counters.

Bug: chromium:774644
Change-Id: I78e5d5c767e22148cb64e8cabe0564e7a13988f5
Reviewed-on: https://chromium-review.googlesource.com/725726Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48751}