- Add JSReceiver::SetIntegrityLevel, with a fast path for regular objects.
- Make Object.{freeze,seal} call this via %Object{Freeze,Seal}, thus no longer
  using broken or deprecated functions from v8natives.js.
- Add JSReceiver::OwnPropertyKeys convenience function.
- Reenable harmony/proxies-hash.js test.

R=rossberg
BUG=v8:1543
LOG=N

Review URL: https://codereview.chromium.org/1489423002

Cr-Commit-Position: refs/heads/master@{#32651}