Only MutableStylePropertySets should be able to have CSSOM wrappers.
<http://webkit.org/b/114595>

Reviewed by Antti Koivisto.

Tighten typing so that only MutableStylePropertySets can have CSSOM wrappers associated.
This allows us to get rid of the out-of-line base class destructor.

* WebCore.exp.in:
* css/StylePropertySet.cpp:
(WebCore::MutableStylePropertySet::~MutableStylePropertySet):
(WebCore::MutableStylePropertySet::cssStyleDeclaration):
(WebCore::MutableStylePropertySet::ensureInlineCSSStyleDeclaration):
* css/StylePropertySet.h:
(StylePropertySet):
(MutableStylePropertySet):
* dom/StyledElement.cpp:
(WebCore::StyledElement::inlineStyleCSSOMWrapper):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@148410 268f45cc-cd09-0410-ab3c-d52691b4dbfc