Call get_(template|stylesheet)_directory_uri() only once in...

Call get_(template|stylesheet)_directory_uri() only once in process_default_headers. props mitchoyoshitaka, fixes #17353.

git-svn-id: https://core.svn.wordpress.org/trunk@18505 1a063a9b-81f0-0310-95a4-ce76da25c4cd