Reason for revert:
No real improvement as we still lack the ability to promote from
scavenges/young gen GCs.

Let's keep this in mind for later.

Original issue's description:
> [heap] Remove border page
>
> A page now belongs either the nursery *or* the intermediate gen. The page that
> contained objects of both spaces is removed in this change.
>
> BUG=chromium:636331
>
> Committed: https://crrev.com/42ece47446f0dbd3779d6e0e00dce97a1931a9f9
> Cr-Commit-Position: refs/heads/master@{#39778}

TBR=ulan@chromium.org,hpayer@chromium.org
BUG=chromium:636331

Review-Url: https://codereview.chromium.org/2383443002
Cr-Commit-Position: refs/heads/master@{#39854}