NewSpace and OldSpace have linear allocation areas, but presently the
implementation doesn't share any code and there are subtle differences.
This CL introduces a superclass 'SpaceWithLinearArea' that will be used
to refactor and share code.

Change-Id: I741e6a6ebb9e75c111287214fd1f555fba62c452
Reviewed-on: https://chromium-review.googlesource.com/809504Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Ali Ijaz Sheikh <ofrobots@google.com>
Cr-Commit-Position: refs/heads/master@{#49890}