The CSA fast path returned null for Proxy.prototype whereas runtime GetProperty
returned undefined. The CL fixes this discrepancy by returning undefined for
both cases and  this makes it complaint with the spec.

Change-Id: I35b75c09dc99e8fd629671e30eacd2cabea8c1d4
Reviewed-on: https://chromium-review.googlesource.com/1145438Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Chandan Reddy <chandanreddy@google.com>
Cr-Commit-Position: refs/heads/master@{#54745}