Enable inobject double fields unboxing for 64-bit archs.

Review URL: https://codereview.chromium.org/753503002

Cr-Commit-Position: refs/heads/master@{#25469}
Revert of Enable inobject double fields unboxing for 64-bit archs. (patchset #3 id:40001 of https://codereview.chromium.org/753503002/)

Reason for revert:
GCMole and gc-stress issues.

Original issue's description:
> Enable inobject double fields unboxing for 64-bit archs.

TBR=verwaest@chromium.org
NOTREECHECKS=true
NOTRY=true

Review URL: https://codereview.chromium.org/751863002

Cr-Commit-Position: refs/heads/master@{#25470}