... and revert "[runtime] Mark maps created as a result of field type or
representation generalization as migration targets." because it regresses
performance of too many map checks in optimized code.

BUG=v8:5444, chromium:680995

Change-Id: I84038d75425df3709a81e5fdcc6c0db32939060d
Reviewed-on: https://chromium-review.googlesource.com/442125Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#43188}