The Great Keys Migration:
This is part of a bigger effort to centralize optimizations for key collections
in a central place. This necessary to avoid the penalty that would be introduced
by fixing shadowed property iteration.

BUG=v8:4758, v8:705
LOG=N

Review-Url: https://codereview.chromium.org/1938413002
Cr-Commit-Position: refs/heads/master@{#35991}