Revert "unified-heap: Enable per default"

This reverts commit 8873329027ef3e919bd8a87118dc13162d2a9d7d.

Reason for revert: Broke leak detector, see https://bugs.chromium.org/p/chromium/issues/detail?id=928781

Original change's description:
> unified-heap: Enable per default
> 
> Enabling unfied heap garbage collections per default.
> 
> Bug: 843903
> Change-Id: I473f85bbe2d6b3fc72051549ffd389896cc488c6
> Reviewed-on: https://chromium-review.googlesource.com/c/1412026
> Reviewed-by: Jochen Eisinger <jochen@chromium.org>
> Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
> Reviewed-by: Kentaro Hara <haraken@chromium.org>
> Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#629116}

TBR=ulan@chromium.org,haraken@chromium.org,mlippautz@chromium.org,jochen@chromium.org

Change-Id: If65954f55b65c9bf51e21d23b6de84282ccb2879
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 843903
Reviewed-on: https://chromium-review.googlesource.com/c/1454918
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#629207}
6 files changed