CSSParser should return ImmutableStylePropertySets.
<http://webkit.org/b/114585>

Reviewed by Anders Carlsson.

The StylePropertySets created by CSSParser were already immutable.
Let this be reflected in the pointer types returned.

* css/CSSParser.cpp:
(WebCore::CSSParser::parseInlineStyleDeclaration):
(WebCore::CSSParser::parseDeclaration):
(WebCore::CSSParser::createStylePropertySet):
* css/CSSParser.h:
(CSSParser):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@148399 268f45cc-cd09-0410-ab3c-d52691b4dbfc