Add a Mac baseline for the test added in r142955.

* platform/mac/TestExpectations:
* platform/mac/svg/filters/filter-hidden-content-expected.png: Added.
* platform/mac/svg/filters/filter-hidden-content-expected.txt: Added.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@143773 268f45cc-cd09-0410-ab3c-d52691b4dbfc