Revert of Better pack fields in Variable (patchset #1 id:1 of https://codereview.chromium.org/2253513002/ )

Reason for revert:
Revert: Breaks ARM build: https://uberchromegw.corp.google.com/i/client.v8.ports/builders/V8%20Arm%20-%20builder/builds/2999

Original issue's description:
> Better pack fields in Variable
>
> This reduces sizeof(Variable) from 64 to 40 on x64
>
> BUG=v8:5209
>
> Committed: https://crrev.com/d84343568047c8621a6b8f88f20a7f34586321b8
> Cr-Commit-Position: refs/heads/master@{#38659}

TBR=marja@chromium.org,jkummerow@chromium.org,verwaest@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=v8:5209

Review-Url: https://codereview.chromium.org/2249203002
Cr-Commit-Position: refs/heads/master@{#38666}