TinyMCE: Fix backspace and line break issues in WebKit. Updates TinyMCE 3.5.8 to include 3 upstream commits.

props azaozz.
for the 3.5 branch.
see [23402] for trunk.

fixes #23337.



git-svn-id: https://core.svn.wordpress.org/branches/3.5@24484 1a063a9b-81f0-0310-95a4-ce76da25c4cd