Everything necessary to implement Object.keys efficiently is already
available in C++ land for quite some time now, and only the thin
JavaScript wrapper was left, so get rid of that as well and move the
whole builtin to C++ instead.

R=yangguo@chromium.org

Review URL: https://codereview.chromium.org/1567963002

Cr-Commit-Position: refs/heads/master@{#33167}