In WebKit2 code including config.h is not needed but check-webkit-style complains about it's absence:
Found header this file implements before WebCore config.h. Should be: config.h, primary header, blank line, and then alphabetically sorted. [build/include_order] 
Additionally we should mention in "WebKit Coding Style Guidelines"
that including config.h is unnecessary.
Sounds like this was fixed in 48638.
*** This bug has been marked as a duplicate of bug 48638 ***