Use DEFINE_FIELD_OFFSET_CONSTANTS to describe AllocationSite fields

Change-Id: I03ae84e1f4720da9da7e2a8a784f56fe6f155e18
Reviewed-on: https://chromium-review.googlesource.com/1095308Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53696}