The Object.keys builtin didn't properly check for
empty_slow_elements_dictionary in addition to empty_fixed_array,
which made it miss the fast-path if you used it in combination with
like Object.freeze or Object.seal. This adds the missing fast-path
support.

Bug: v8:6767
Change-Id: I48e43b2ee51eb2d48446c45748401af096020bb7
Reviewed-on: https://chromium-review.googlesource.com/663539Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47985}