Update obtain-a-blob-object to handle top-level self-fetch

Bypasses blob URL partitioning for fetches that are a top-level
self-fetch. For more info, see:
#210

Author: recvfrom

index.bs

@@ -1526,11 +1526,13 @@ an [=empty host=], and a [=url/path=] consisting of one element itself also a [=
 
 <div algorithm="obtainBlobObject">
 To <dfn export id=blob-url-obtain-object>obtain a blob object</dfn> given a [=blob URL entry=]
-|blobUrlEntry| and an [=environment settings object=] or the string "`navigation`" |environment|,
-perform the following steps. They return an [=blob URL entry/object=].
+|blobUrlEntry| and an [=environment settings object=], the string "`navigation`", or the string
+"`top-level-self-fetch`" |environment|, perform the following steps. They return an [=blob URL entry/object=].
 
 1. Let |isAuthorized| be true.
-1. If |environment| is not the string "`navigation`", then set |isAuthorized| to the result of [=checking for same-partition blob URL usage=] with |blobUrlEntry| and |environment|.
+1. If |environment| is not the string "`navigation`" and not the string "`top-level-self-fetch`",
+   then set |isAuthorized| to the result of [=checking for same-partition blob URL usage=] with
+   |blobUrlEntry| and |environment|.
 1. If |isAuthorized| is false, then return failure.
 1. Return |blobUrlEntry|'s [=blob URL entry/object=].