FormatJson: Skip whitespace cleanup when unnecessary

The patch[1] for PHP bug 66021[2], which removes the same undesirable
whitespace that WS_CLEANUP_REGEX does, has been merged into php-src.
Subsequent PHP versions having the patch shouldn't have to take the
10-20% performance hit from that workaround.

[1]: https://github.com/php/php-src/commit/82a4f1a1a287
[2]: https://bugs.php.net/bug.php?id=66021

Change-Id: I717a0e164952cc6ace104f13f6236e86c4ab8b58
Merge "FormatJson: Skip whitespace cleanup when unnecessary"