Anonymous iframe (WIP)

Explainer & specs (WIP)
https://arthursonzogni.github.io/anonymous-iframe/#explainer

Summary:
- Add `partition-nonce` to network-partition-key. The value is filled
  from the HTML specification.

Anonymous iframe require updating several specifications:
- HTML => whatwg/html#7695
- Fetch => (this)
- CHIPS (cookie-having-independent-partition-state) => XXX
- Storage-partitioning => XXX

fetch.bs

@@ -2652,8 +2652,12 @@ details of reused connections are not exposed and time values are coarsened.
 
 <h3 id=network-partition-keys>Network partition keys</h3>
 
-<p>A <dfn>network partition key</dfn> is a tuple consisting of a <a for=/>site</a> and null or
-an <a>implementation-defined</a> value.
+<p>A <dfn>network partition key</dfn> is a tuple consisting of:</p>
+<ul class="brief">
+  <li><p>a <a for=/>site</a>.</p>
+  <li><p>null or an <a>implementation-defined</a> value.</p>
+  <li><p>null or a nonce.</p>
+</ul>
 
 <p>To
 <dfn export lt="determine the network partition key|determining the network partition key">determine the network partition key</dfn>,
@@ -2671,13 +2675,14 @@ given an <a for=/>environment</a> <var>environment</var>, run these steps:
  <li><p>Let <var>topLevelSite</var> be the result of <a lt="obtain a site">obtaining a site</a>,
  given <var>topLevelOrigin</var>.
 
- <li>
-  <p>Let <var>secondKey</var> be null or an <a>implementation-defined</a> value.
+ <li><p>Let <var>secondKey</var> be null or an <a>implementation-defined</a> value.
+
+ <p class=XXX>The second key is intentionally a little vague as the finer points are still
+ evolving. See <a href=https://github.com/whatwg/fetch/issues/1035>issue #1035</a>.
 
-  <p class=XXX>The second key is intentionally a little vague as the finer points are still
-  evolving. See <a href=https://github.com/whatwg/fetch/issues/1035>issue #1035</a>.
+ <li><p>Let <var>nonce</var> be <var>environment</var>'s <a for="environment">partition-nonce</a></p>
 
- <li><p>Return (<var>topLevelSite</var>, <var>secondKey</var>).
+ <li><p>Return (<var>topLevelSite</var>, <var>secondKey</var>, <var>nonce</var>).
 </ol>
 
 <p>To