JSProxy::HasProperty was missing an early "return Nothing<bool>".
KeyAccumulator's FilterProxyKeys() didn't handle SKIP_STRINGS correctly.

BUG=v8:1543
LOG=n
R=cbruni@chromium.org

Review URL: https://codereview.chromium.org/1505253002

Cr-Commit-Position: refs/heads/master@{#32699}