When deleting the most recently added fast property from an object
by undoing its last map transition, we must clear any recorded slots.
This can only be done in C++, so this functionality must move out
of the stub.
Also update a CHECK in the JSObject verifier to allow backing stores
sticking around after such property deletions.

BUG=chromium:716912,chromium:714981

Review-Url: https://codereview.chromium.org/2854373002
Cr-Commit-Position: refs/heads/master@{#45069}