Having beefed up GetKeys() to support everything, use it for everything now.

This fixes Object.getOwnPropertyNames and Object.getOwnPropertySymbols for
Proxies, and gets rid of a bunch of code duplication.

BUG=v8:1543
LOG=n

Review URL: https://codereview.chromium.org/1498593006

Cr-Commit-Position: refs/heads/master@{#32620}