Revert of Use in-object fields instead of private symbols for regexp slots. (patchset #4 id:60001 of https://codereview.chromium.org/1428203003/ )

Reason for revert:
A clean revert of https://codereview.chromium.org/1419823010/ was not possible because of this CL. Thus, I am also reverting this CL.

Original issue's description:
> Use in-object fields instead of private symbols for regexp slots.
>
> R=bmeurer@chromium.org
>
> Committed: https://crrev.com/5a1e42c039ac3379ebe1e7e34fb8163e1ec1493e
> Cr-Commit-Position: refs/heads/master@{#31791}

TBR=bmeurer@chromium.org,yangguo@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true

Review URL: https://codereview.chromium.org/1421593009

Cr-Commit-Position: refs/heads/master@{#31800}