Reland of Put RegExp js code in strict mode (patchset #2 id:20001 of https://codereview.chromium.or… (patchset #2 id:20001 of https://codereview.chromium.org/2112713003/ )

Reason for revert:
With fixes for frozen RegExps in https://codereview.chromium.org/2339443002 , it should be web-compatible to put RegExps in strict mode again, per spec.

Original issue's description:
> Revert of Put RegExp js code in strict mode (patchset #2 id:20001 of https://codereview.chromium.org/1776883005/ )
>
> Reason for revert:
> Found to break SAP Web IDE, and these semantics are not shipped in any other browser.
> Revert to legacy semantics while assessing web compatibility.
>
> BUG=chromium:624318
>
> Original issue's description:
> > Put RegExp js code in strict mode
> >
> > src/js/regexp.js was one of the few files that was left in sloppy
> > mode. The ES2017 draft specification requires that writes to
> > lastIndex throw when the property is non-writable, and test262
> > tests enforce this behavior. This patch puts that file in strict
> > mode.
> >
> > BUG=v8:4504
> > R=yangguo@chromium.org
> > LOG=Y
> >
> > Committed: https://crrev.com/80b1b2a45bbd9bf3d08e4e6516acfaaa8f438213
> > Cr-Commit-Position: refs/heads/master@{#34801}
>
> TBR=yangguo@chromium.org,adamk@chromium.org
>
> Committed: https://crrev.com/34880eb3dcf7492d44c0a3b45b6c888189f2c3c3
> Cr-Commit-Position: refs/heads/master@{#37449}

TBR=adamk@chromium.org,yangguo@chromium.org
# Not skipping CQ checks because original CL landed more than 1 days ago.
BUG=chromium:624318

Review-Url: https://codereview.chromium.org/2344773002
Cr-Commit-Position: refs/heads/master@{#39456}