Sync toWebKitCSSShaderValue() behavior with other toCSSFooValue()
https://bugs.webkit.org/show_bug.cgi?id=121886

Reviewed by Darin Adler.

toWebKitCSSShaderValue() behavior is different from other to CSSFooValue().
It shouldn't return null. The type checking should be done before doing type cast.

* css/StyleResolver.cpp:
(WebCore::StyleResolver::createCustomFilterOperationWithInlineSyntax):
* css/WebKitCSSShaderValue.h:
(WebCore::toWebKitCSSShaderValue):


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@156528 268f45cc-cd09-0410-ab3c-d52691b4dbfc