For now, we revoke a proxy by setting its handler to null (as in the spec).

Change the "target" field from Object to JSReceiver as there's no point in
allowing more.

R=jkummerow@chromium.org, rossberg
BUG=v8:1543
LOG=n

Review URL: https://codereview.chromium.org/1496243003

Cr-Commit-Position: refs/heads/master@{#32608}