[builtins] Speedup Object.keys by adding a fast path for objects without elements, interceptors, ...

This speeds up the for-of-object benchmark at
http://kpdecker.github.io/six-speed/ by >2x.

BUG=

Review URL: https://codereview.chromium.org/1682873003

Cr-Commit-Position: refs/heads/master@{#33867}