Make WebKitCSSShaderValue inherit directly from CSSValue.

Make WebKitCSSShaderValue inherit directly from CSSValue.
<http://webkit.org/b/79144>

Reviewed by Antti Koivisto.

WebKitCSSShaderValue was inheriting from CSSPrimitiveValue but only used
it for storing the shader URL. Moved that to a String member instead.

WebKitCSSShaderValues didn't even return true for isPrimitiveValue() so
there should be no behavior change whatsoever here.

* css/WebKitCSSShaderValue.cpp:
(WebCore::WebKitCSSShaderValue::WebKitCSSShaderValue):
(WebCore::WebKitCSSShaderValue::cachedShader):
(WebCore::WebKitCSSShaderValue::customCssText):
* css/WebKitCSSShaderValue.h:
(WebKitCSSShaderValue):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@108396 268f45cc-cd09-0410-ab3c-d52691b4dbfc