Revert of Temporarily disable double fields unboxing. (patchset #1 id:1 of https://codereview.chromium.org/928733003/)

Reason for revert:
Preparing to land a fix.

Original issue's description:
> Temporarily disable double fields unboxing.
>
> Committed: https://crrev.com/209cf09ac9e36c1a24cdfa918bc579a4671c6842
> Cr-Commit-Position: refs/heads/master@{#26727}

TBR=jkummerow@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true

Review URL: https://codereview.chromium.org/960173002

Cr-Commit-Position: refs/heads/master@{#26876}