Analog to other interceptors, intercept the DefineProperty
call only after obtaining the property descriptor.

This behavior allows us to mirror calls on a sandboxed object
as it is needed in Node. See for example
https://github.com/nodejs/node/pull/13265

Bug: 
Change-Id: I73b8f8908d13473939b37fb6727858d0bee6bda3
Reviewed-on: https://chromium-review.googlesource.com/725295Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Franziska Hinkelmann <franzih@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48683}