Since we never extract pools from a {DisjointAllocationPool}, the
{Allocate} method can just return an {AddressRange}, and also {Merge}
just needs to merge a single {AddressRange}.

Drive-by: Make {AddressRange} a proper struct, for DCHECKs and better
accessors.

R=mstarzinger@chromium.org

Bug: v8:7754
Change-Id: I19fd02b2c6d8eb5316a5e994835b89be9cfa792b
Reviewed-on: https://chromium-review.googlesource.com/1090723
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53610}