This is a reland of 8dd405e3. The MSAN
failures were unrelated and should be fixed by 75c1c6c6.

Original change's description:
> [heap] introduce SpaceWithLinearArea class
>
> 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/809504
> Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
> Commit-Queue: Ali Ijaz Sheikh <ofrobots@google.com>
> Cr-Commit-Position: refs/heads/master@{#49890}

Change-Id: I275bdc881b1b81f3ae3c7d36e919981ac4331315
Reviewed-on: https://chromium-review.googlesource.com/812324Reviewed-by: Ali Ijaz Sheikh <ofrobots@google.com>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49926}