Reserve/free quota for fetchLater()

The logic for deferred fetching (the `fetchLater` function), as
defined in the fetch spec, specifies a "quota" which is shared
with between a document and its direct same-origin descendants.

For this logic to work in a secure way, the quota needs to be:
- reserved when a frame-initiated navigation starts. This way,
  the container document can only reserve quota based on URLs
  it knows it navigates to.
- freed if the document ends up being same origin with its
  container, upon document creation.
  This ensures quota is handled correctly in the case of
  redirects.

This PR adds those two calls:
- Call "reserve" on navigation, based on `sourceDocument`.
- Call "potentially free" on document creation.

Builds on whatwg/fetch#1647, where
the quota logic itself is defined.

Author: noamr

source

@@ -2624,6 +2624,8 @@ a.setAttribute('href', 'https://example.com/'); // change the content attribute
      <li><dfn data-x-href="https://fetch.spec.whatwg.org/#byte-sequence-as-a-body">as a body</dfn></li>
      <li><dfn data-x="response-body-info" data-x-href="https://fetch.spec.whatwg.org/#response-body-info">response body info</dfn></li>
      <li><dfn data-x-href="https://fetch.spec.whatwg.org/#resolve-an-origin">resolve an origin</dfn></li>
+     <li><dfn data-x-href="https://fetch.spec.whatwg.org/#reserve-deferred-fetch-quota">reserve deferred fetch quota</dfn></li>
+     <li><dfn data-x-href="https://fetch.spec.whatwg.org/#reserve-deferred-fetch-quota">potentially free deferred fetch quota</dfn></li>
      <li>
       <dfn data-x="concept-response"
       data-x-href="https://fetch.spec.whatwg.org/#concept-response">response</dfn> and its
@@ -102327,6 +102329,11 @@ location.href = '#foo';</code></pre>
     data-x="navigate-fragid">navigate to a fragment</span> path.</p>
    </li>
 
+   <li><p>If <var>sourceDocument</var> is <var>navigable</var>'s <span
+   data-x="nav-container-document">container document</span>, then <span>reserve deferred fetch
+   quota</span> for <var>navigable</var>'s <span data-x="nav-container">container</span> given
+   <var>url</var>'s <span data-x="concept-url-origin">origin</span>.</p></li>
+
    <li>
     <p><span>In parallel</span>, run these steps:</p>
 
@@ -107235,6 +107242,8 @@ location.href = '#foo';</code></pre>
    <var>navigationParams</var>'s <span data-x="navigation-params-response">response</span>, and
    "<code data-x="">pre-media</code>".</p></li>
 
+   <li><p><span>Potentially free deferred fetch quota</span> for <var>document</var>.</p></li>
+
    <li><p>Return <var>document</var>.</p></li>
   </ol>