The build script copies public WebCore header files to WebKitBuild/include. Apparently, when a header is removed from source tree, it is not reflected in WebKitBuild/include during an incremental build. Hence, if there's still a file in WebKit that depends on the removed header, the compilation does not fail.
See http://trac.webkit.org/changeset/68384 & http://trac.webkit.org/changeset/68371 for example of this happening (the bots did not indicate a build failure).
Thank you for filing. There are lots of windows build issues. We need to slowly find them all and fix them all to keep our sanity. :)