Implement BLOB in a working way.

* The previous implementation made no sense, and because of leftover
ENABLE(BLOB) preprocessor guards parts of it weren't even compiled in.
* Fixes the crash in #11021.
* This is still incomplete, only blobs with a single file are supported
(no composite blobs, no data blob items). Added debugger() calls for the
missing cases, please report if you know a website that hits them.