script: fix BorrowError in (new Blob).slice(0,0).text()
When getting the text of a sliced Blob, we call GlobalScope.get_blob_url_id, which borrows the blob_state field mutably and calls the get_blob_size method (aka [Blob#size](https://w3c.github.io/FileAPI/#dfn-size)), which tries to borrow the same field immutably, causing a panic.
This patch inlines the relevant parts of get_blob_size into get_blob_url_id, so we can reuse the mutable borrow.
* /FileAPI/Blob-methods-from-detached-frame.html was 4/4 FAIL, will be 4/4 PASS once #29396 also lands
* /fetch/api/basic/scheme-blob.sub.any.html was CRASH, now 10/17 PASS and 7/17 FAIL
* /fetch/api/basic/scheme-blob.sub.any.worker.html was CRASH, now 10/17 PASS and 7/17 FAIL
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- [x] These changes fix#29450
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