Add NavigationTransition.to, exposing the current navigate event's destination

Add `NavigationTransition.to`, exposing the current `navigate` event's destination.

This was not necessary before as navigations were committed synchronously, but with precommit handlers this
exposes information about an ongoing navigation.

The destination is set when the `NavigationTransition` object is created, and remains stable throughout.

Fixes #11690.

Author: noamr

source

@@ -100332,6 +100332,7 @@ const p2 = navigation.navigate(url2).finished;</code></pre>
 interface <dfn interface>NavigationTransition</dfn> {
   readonly attribute <span>NavigationType</span> <span data-x="dom-NavigationTransition-navigationType">navigationType</span>;
   readonly attribute <span>NavigationHistoryEntry</span> <span data-x="dom-NavigationTransition-from">from</span>;
+  readonly attribute <span>NavigationDestination</span> <span data-x="dom-NavigationTransition-to">to</span>;
   readonly attribute <span data-x="idl-Promise">Promise</span>&lt;undefined> <span data-x="dom-NavigationTransition-committed">committed</span>;
   readonly attribute <span data-x="idl-Promise">Promise</span>&lt;undefined> <span data-x="dom-NavigationTransition-finished">finished</span>;
 };</code></pre>
@@ -100366,6 +100367,12 @@ interface <dfn interface>NavigationTransition</dfn> {
    useful to compare against <code
    data-x="dom-Navigation-currentEntry">navigation.currentEntry</code>.</p></dd>
 
+   <dt><code data-x=""><span data-x="dom-navigation">navigation</span>.<span data-x="dom-Navigation-transition">transition</span>.<span subdfn data-x="dom-NavigationTransition-to">to</span></code></dt>
+   <dd><p>The <code>NavigationDestination</code> of the transition. This can be useful as a way to
+   reflect a current navigation's destination's <code
+   data-x="dom-NavigationDestination-url">url</code> without having to listen to the <code
+   data-x="event-navigate">navigate</code> event.</p></dd>
+
    <dt><code data-x=""><span data-x="dom-navigation">navigation</span>.<span data-x="dom-Navigation-transition">transition</span>.<span subdfn data-x="dom-NavigationTransition-committed">committed</span></code></dt>
    <dd><p>A promise which fulfills once the <code
    data-x="dom-Navigation-currentEntry">navigation.currentEntry</code> and <span
@@ -100401,6 +100408,10 @@ interface <dfn interface>NavigationTransition</dfn> {
   data-x="concept-NavigationTransition-from">from entry</dfn>, which is a
   <code>NavigationHistoryEntry</code>.</p>
 
+  <p>Each <code>NavigationTransition</code> has an associated <dfn
+  data-x="concept-NavigationTransition-destination">destination</dfn>, which is a
+  <code>NavigationDestination</code>.</p>
+
   <p>Each <code>NavigationTransition</code> has an associated <dfn
   data-x="concept-NavigationTransition-committed">committed promise</dfn>, which is a promise.</p>
 
@@ -100420,12 +100431,21 @@ interface <dfn interface>NavigationTransition</dfn> {
   <span>this</span>'s <span data-x="concept-NavigationTransition-from">from entry</span>.</p>
   </div>
 
+  <div algorithm>
+  <p>The <dfn attribute for="NavigationTransition"><code
+  data-x="dom-NavigationTransition-to">to</code></dfn> getter steps are to return
+  <span>this</span>'s <span
+  data-x="concept-NavigationTransition-destination">destination</span>.</p>
+  </div>
+
   <div algorithm>
   <p>The <dfn attribute for="NavigationTransition"><code
   data-x="dom-NavigationTransition-committed">committed</code></dfn> getter steps are to return
   <span>this</span>'s <span data-x="concept-NavigationTransition-committed">committed
   promise</span>.</p>
+  </div>
 
+  <div algorithm>
   <p>The <dfn attribute for="NavigationTransition"><code
   data-x="dom-NavigationTransition-finished">finished</code></dfn> getter steps are to return
   <span>this</span>'s <span data-x="concept-NavigationTransition-finished">finished
@@ -101716,6 +101736,9 @@ interface <dfn interface>NavigationDestination</dfn> {
        <dt><span data-x="concept-NavigationTransition-from">from entry</span></dt>
        <dd><var>fromNHE</var></dd>
 
+       <dt><span data-x="concept-NavigationTransition-destination">destination</span></dt>
+       <dd><var>event</var>'s <code data-x="dom-NavigateEvent-destination">destination</code></dd>
+
        <dt><span data-x="concept-NavigationTransition-committed">committed promise</span></dt>
        <dd>a new promise created in <var>navigation</var>'s <span
        data-x="concept-relevant-realm">relevant realm</span></dd>