StylePropertySet::getPropertyShorthand() should return a String.
<http://webkit.org/b/115213>

Reviewed by Anders Carlsson.

Return a String directly from getPropertyShorthand() instead of forcing clients
to call getPropertyNameString().

* css/PropertySetCSSStyleDeclaration.cpp:
(WebCore::PropertySetCSSStyleDeclaration::getPropertyShorthand):
* css/StylePropertySet.cpp:
(WebCore::StylePropertySet::getPropertyShorthand):
* css/StylePropertySet.h:
(StylePropertySet):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@149157 268f45cc-cd09-0410-ab3c-d52691b4dbfc