Shortcode JS: Avoid errors when an escaped shortcode includes a newline between brackets.

Merges [28223] to the 3.9 branch.

props gcorne.
props Clorith, jorbin for tests.
fixes #27907.


git-svn-id: https://develop.svn.wordpress.org/branches/3.9@28224 602fd350-edb4-49c9-b593-d223f7449a82