Prior to this patch, regular expression objects with a monkeypatched
`toString` were printed using the `toString` result value, rather than
actually representing the regular expression’s contents.

    const re = /./;
    re.toString = () => 'whoops!';
    console.log(re);
    // → logs 'whoops!'

Now that `v8::RegExp::GetSource` properly escapes special characters in
the source pattern [1], just like `RegExp#toString`, there is no longer
any reason to avoid it.

[1]: https://chromium-review.googlesource.com/c/v8/v8/+/2900737

Bug: v8:11693
Change-Id: I9a69cdb6813f76b669bdc24e4823c6d261f2ae73
Fixed: v8:11836
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2928188Reviewed-by: Philip Pfaffe <pfaffe@chromium.org>
Commit-Queue: Mathias Bynens <mathias@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74862}