Add history.index

Closes whatwg#2710.

Author: domenic

source

@@ -80288,6 +80288,7 @@ interface <dfn>BarProp</dfn> {
 
 [Exposed=Window]
 interface <dfn>History</dfn> {
+  readonly attribute unsigned long <span data-x="dom-history-index">index</span>;
   readonly attribute unsigned long <span data-x="dom-history-length">length</span>;
   attribute <span>ScrollRestoration</span> <span data-x="dom-history-scroll-restoration">scrollRestoration</span>;
   readonly attribute any <span data-x="dom-history-state">state</span>;
@@ -80300,7 +80301,16 @@ interface <dfn>History</dfn> {
 
   <dl class="domintro">
 
-   <dt><var>window</var> . <code subdfn data-x="dom-history">history</code> . <code subdfn data-x="dom-history-length">length</code></dt>
+   <dt><var>window</var> . <code subdfn data-x="dom-history">history</code> . <code subdfn data-x="dom-history-index">index</code></dt>
+
+   <dd>
+
+    <p>Returns the index of the <span>current entry of the joint session history</span> within the
+    <span>joint session history</span>.</p>
+
+   </dd>
+
+   <dt><var>window</var> . <code data-x="dom-history">history</code> . <code subdfn data-x="dom-history-length">length</code></dt>
 
    <dd>
 
@@ -80407,6 +80417,13 @@ interface <dfn>History</dfn> {
 
   <div w-nodev>
 
+  <p>The <dfn><code data-x="dom-history-index">length</code></dfn> attribute of the
+  <code>History</code> interface, on getting, must return the index of the <span>current entry of
+  the joint session history</span> within the <span>top-level browsing context</span>'s <span>joint
+  session history</span>. If this <code>History</code> object is associated with a
+  <code>Document</code> that is not <span>fully active</span>, getting must instead throw a
+  <span>"<code>SecurityError</code>"</span> <code>DOMException</code>.</p>
+
   <p>The <dfn><code data-x="dom-history-length">length</code></dfn> attribute of the
   <code>History</code> interface, on getting, must return the number of entries in the
   <span>top-level browsing context</span>'s <span>joint session history</span>. If this