Use convenient macros for accessing bit fields.

Bug: v8:6470
Change-Id: Iada9779ce56c7ca2e8b6a9617c236e294db7325e
Reviewed-on: https://chromium-review.googlesource.com/527432
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45804}