Support obtaining a network partition key from an environment

I noticed in whatwg/storage#137 that the current setup is slightly bogus.

Author: annevk

fetch.bs

@@ -2614,14 +2614,14 @@ an <a>implementation-defined</a> value.
 
 <p>To
 <dfn export lt="determine the network partition key|determining the network partition key">determine the network partition key</dfn>,
-given an <a for=/>environment settings object</a> <var>settings</var>, run these steps:
+given an <a for=/>environment</a> <var>environment</var>, run these steps:
 
 <ol>
- <li><p>Let <var>topLevelOrigin</var> be <var>settings</var>'s
+ <li><p>Let <var>topLevelOrigin</var> be <var>environment</var>'s
  <a for="environment">top-level origin</a>.
 
  <li><p>If <var>topLevelOrigin</var> is null, then set <var>topLevelOrigin</var> to
- <var>settings</var>'s <a for="environment">top-level creation URL</a>'s <a for=url>origin</a>.
+ <var>environment</var>'s <a for="environment">top-level creation URL</a>'s <a for=url>origin</a>.
 
  <li><p>Assert: <var>topLevelOrigin</var> is an <a for=/>origin</a>.