In general, deleting a property from a fast-properties object
requires transitioning the object to dictionary mode. However,
when the most-recently-added property is deleted, we can simply
roll back the last map transition that the object went through.

This is a performance experiment: it should make things faster,
but if it turns out to have more negative than positive impact,
we will have to revert it.

TBR=bmeurer@chromium.org (just adding a comment)

Previously reviewed at https://codereview.chromium.org/2830093002
Previously landed as 98acfb36 / r44799

Review-Url: https://codereview.chromium.org/2840583002
Cr-Commit-Position: refs/heads/master@{#44808}