.. by using macros for defining field offsets.

Bug: v8:5799
Change-Id: Id76e81bedb8f348b2efaa1d553bebac0ff90b474
Reviewed-on: https://chromium-review.googlesource.com/768382Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49351}