Anonymous iframe

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

This patch adds the `environment`'s `partition-nonce` to the
`storage-key`.
  from the HTML specification.

Anonymous iframe require updating several specifications:
- HTML => whatwg/html#7695
- Fetch => whatwg/fetch#1416
- Storage => (this)
- CHIPS/Cookies => XXX

storage.bs

@@ -198,8 +198,11 @@ anticipated that some APIs will be applicable to both <a>storage types</a> going
 
 <h3 id=storage-keys>Storage keys</h3>
 
-<p>A <dfn>storage key</dfn> is a <a>tuple</a> consisting of an <dfn for="storage key">origin</dfn>
-(an <a for=/>origin</a>). [[!HTML]]
+<p>A <dfn>storage key</dfn> is a <a>tuple</a> consisting of:
+<ul>
+  <li><p>An <dfn for="storage key">origin</dfn> (an <a for=/>origin</a>). [[!HTML]]</p>
+  <li><p>A <dfn for="storage key">nonce</dfn> (an identifier or null). [[!HTML]]
+</ul>
 
 <p class=XXX>This is expected to change; see
 <a href="https://privacycg.github.io/storage-partitioning/">Client-Side Storage Partitioning</a>.
@@ -228,7 +231,10 @@ anticipated that some APIs will be applicable to both <a>storage types</a> going
  <a>environment settings object</a>; otherwise <var>environment</var>'s
  <a for=environment>creation URL</a>'s <a for=url>origin</a>.
 
- <li><p>Return a <a>tuple</a> consisting of <var>origin</var>.
+ <li><p>Let <var>nonce</var> be <var>environment</var>'s <a for="environment settings
+ object">partition nonce</a>.
+
+ <li><p>Return a <a>tuple</a> consisting of <var>origin</var> and <var>nonce</var>.
 </ol>
 
 <p>To determine whether a <a>storage key</a> <var>A</var>