[GTK] Implement sizes attribute for link tag
https://bugs.webkit.org/show_bug.cgi?id=125775

Reviewed by Martin Robinson.

* bindings/gobject/WebKitDOMCustom.cpp:
(webkit_dom_html_link_element_set_sizes): Add custom setter implementation.
* bindings/gobject/WebKitDOMCustomUnstable.h:
* bindings/js/JSHTMLLinkElementCustom.cpp:
(WebCore::JSHTMLLinkElement::sizes): Remove custom getter implementation.
* bindings/scripts/CodeGeneratorGObject.pm:
(HasUnstableCustomAPI): Mark WebKitDOMHTMLLinkElement as having custom unstable API.
* html/HTMLLinkElement.idl: Mark sizes attribute as CustomSetter
instead of Custom, since the getter can be autogenerated, and make
it available to the GObject DOM bindings too.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@177143 268f45cc-cd09-0410-ab3c-d52691b4dbfc