Add the JavaScript API

spec/index.bs

@@ -333,6 +333,22 @@ Issue: Should the header checking be integrated into the fetch spec instead,
 User agents must also clear the [=IDP Sign-in Status map=] when the user clears
 cookies or stored site data.
 
+IDPs can also use a JavaScript API to update the stored sign-in status:
+
+<pre class="idl">
+  [Exposed=Window, SecureContext]
+  interface IdentityProvider {
+    static void recordSignIn();
+    static void recordSignOut();
+  };
+</pre>
+
+When {{IdentityProvider/recordSignIn}} or {{IdentityProvider/recordSignOut}} is
+called, the user agent [=map/set|sets=] the entry in the [=IDP Sign-in Status
+Map=] with the [=map/key=] being [=this=]'s [=relevant global object=]'s
+[=associated document=]'s [=/origin=] and the value being [=signed-in=] (for
+`recordSignIn`) or [=signed-out=] (for `recordSignOut`).
+
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 ## The State Machine ## {#browser-api-state-machine}
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