For this to work, I had to move PropertyCell out of objects.h too, since
otherwise there would be an inl include cycle which makes the code not compile.

BUG=v8:5402,v8:8238

Change-Id: I3233f86b68c1e2fd32d135fcf0bbba8101af8cb2
Reviewed-on: https://chromium-review.googlesource.com/c/1356510Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58004}