When removing the dependencies on CoreFoundation.dll, I noticed that LegacyWebArchive is implemented entirely in CoreFoundation types. Since the Cairo build doesn't currently use this, and if it did we would re-implement using other data types, it should be excluded from the Cairo build.
Now that I've got a working CoreFoundation analog (CFLite), it turns out that the Cairo build actually *will* continue using these types. So, this bug could be closed, as it is not really true anymore.
Closing bug per Comment #1.