We already use {base::AddressRegion} in some places, while other places
still receive {Address} and {size_t}. Those are often created from an
existing {base::AddressRegion}, hence pass that on explicitly.

Drive-by: Rename {AssignRanges} to {AssignRange}.
Drive-by^2: Return {base::AddressRegion} by value (it is trivially
            copyable and small).

R=mstarzinger@chromium.org

Bug: v8:9183
Change-Id: Ia9f26cc96e084922f5e27d879209ee4c79c63fee
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1613242
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61578}