Do not overwrite handle values in AddNamedProperty which could cause
invalid handles in combination with CanonicalHandleScope.

Bug: chromium:802333
Change-Id: I373ab60579901bba65336ae3814e466e07392e22
Reviewed-on: https://chromium-review.googlesource.com/873032Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50890}