For unknown Argument object Maps we have to expect that constants fields
are kept on the Map.

Bug: chromium:729597
Change-Id: I110f77455ce434a431c8de27d021b1a5deb86f30
Reviewed-on: https://chromium-review.googlesource.com/532900Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45935}