Revert of Remove RegExp.multiline accessors. (patchset #3 id:40001 of https://codereview.chromium.org/1410993008/ )

Reason for revert:
[Sheriff] Causes layout test failures.

Original issue's description:
> Remove RegExp.multiline accessors.
>
> This is non-standard and not even documented on MDN.
>
> On Firefox, setting RegExp.multiline to true adds the multiline flag to all
> newly created RegExp objects (both from constructor and from literal).
>
> In V8 this has no effect.
>
> Source archaelogy shows that this is from the initial commit.
>
> R=bmeurer@chromium.org
>
> Committed: https://crrev.com/e8f752ce0c2a488e88cd87fe75f3907b4303d0a0
> Cr-Commit-Position: refs/heads/master@{#31673}

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

Review URL: https://codereview.chromium.org/1431433004

Cr-Commit-Position: refs/heads/master@{#31679}