Files in WebKit/mac should not be used by other platforms. This is error prone as evidenced by bug 32753. Please break this dependence.
I understand the frustration, but I don't actually think that this is possible. If it were possible to do without, I don't think it'd be part of our build. Mac Chromium didn't always use files from WebKit/mac, but when we added it, I made sure we answered the hard questions like "is there any way around it?" I don't remember the details, though.
Bug 32753 doesn't seem all that bad, and neither does its fix.
I'd call this invalid or wontfix or whatever, but I'm not special enough.