This CL fixes a long-standing bug with Object.keys where the enumerability
check was omitted if the [ownKeys] trap is not present. The only distinction the
KeyAccumulator needs is whether it collects keys for for-in (is_for_in_) or not.
ForInFilter performs a separate step to filter out non-enumerable keys later-on
while in all the other use-cases we have to filter keys.

BUG=v8:1543, v8:5250

Review-Url: https://codereview.chromium.org/2176113009
Cr-Commit-Position: refs/heads/master@{#38199}