Revert of heap: make array buffer maps disjoint

Revert of heap: make array buffer maps disjoint (patchset #8 id:140001 of https://codereview.chromium.org/1316873004/ )

Reason for revert:
Precautionary revert. The change is incomplete.

Original issue's description:
> heap: make array buffer maps disjoint
>
> Remove intersection from the `std::map`s representing current live
> ArrayBuffers. While being simpler to understand, it poses significant
> performance issue for the active ArrayBuffer users (like node.js).
>
> Store buffers separately, and process them together during mark-sweep phase.
>
> The results of benchmarks are:
>
> $ ./node-slow bench && ./node-fast bench
> 4997.4 ns/op
> 4685.7 ns/op
>
> NOTE: `fast` - was a patched node.js, `slow` - unpatched node.js with vanilla v8.
>
> BUG=
>
> Committed: https://crrev.com/9e3676da9ab1aaf7de3e8582cb3fdefcc3dbaf33
> Cr-Commit-Position: refs/heads/master@{#30495}

TBR=hpayer@chromium.org,fedor@indutny.com
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=

Committed: https://crrev.com/bb7b53110e3478a38b4d2429aaab6576359deda1
Cr-Commit-Position: refs/heads/master@{#30502}

[Michael Lippautz, 2015-09-01 09:58:08]

Created Revert of heap: make array buffer maps disjoint

[commit-bot: I haz the power, 2015-09-01 09:58:19]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1302233007/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1302233007/1

[commit-bot: I haz the power, 2015-09-01 09:58:32]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-09-01 09:58:51]

Patchset 1 (id:??) landed as
https://crrev.com/bb7b53110e3478a38b4d2429aaab6576359deda1
Cr-Commit-Position: refs/heads/master@{#30502}