Developing for Webkit can be a big headache. The sheer size & complexity of the architecture is mind boggling.
We were facing this error and just could find any way to fix it.
d:\XXX\WebCore\DerivedSources\CSSValueKeywords.h(28) : see declaration of 'CSSValue' d:\XXX\WebCore\DerivedSources\CSSValueKeywords.h(286) : error C2370: 'CSSValue' : redefinition; different storage class
The one solution mentioned in the code is to get the full code and do a rebuild :).
But we found out that the problem lies in CSSValueKeywords.in. If there are any empty lines in that file then it would generate duplicate CSSValue variables.
So just remove all those empty lines from CSSValueKeywords.in and do a rebuild and the problem should be solved.