add 'midi' feature

Controls access to requestMIDIAccess method and defaults to enabled for
top-level browsing context. Nested contexts are default off; parent
browsing context must explicitly enable the feature.

index.html

@@ -442,6 +442,15 @@ <h2>Features</h2>
             <td>`null`</td>
             <td>Controls access to [Geolocation interface].</td>
           </tr>
+          <tr>
+            <td>
+              <a>midi</a>
+            </td>
+            <td>`self`</td>
+            <td>`null`</td>
+            <td>`null`</td>
+            <td>Controls access to [requestMIDIAccess method].</td>
+          </tr>
           <tr>
             <td>
               <a>notifications</a>
@@ -556,6 +565,22 @@ <h2><dfn>`geolocation`</dfn></h2>
           </li>
         </ol>
       </section>
+      <section>
+        <h2><dfn>`midi`</dfn></h2>
+        <pre class="idl">
+partial interface Navigator {
+  [Feature=midi] Promise<MIDIAccess> requestMIDIAccess (optional MIDIOptions options);
+};</pre>
+        <p>The <a>midi</a> keyword controls whether the [requestMIDIAccess method] ([[!WEBMIDI]]) is [exposed] for [current global
+        object].</p>
+        <ol>
+          <li>The default <a>enable policy</a> is `self` for [top-level
+          browsing context], and `null` for [nested browsing context].
+          </li>
+          <li>The default <a>disable policy</a> is `null`.
+          </li>
+        </ol>
+      </section>
       <section>
         <h2><dfn>`notifications`</dfn></h2>
         <pre class="idl">
@@ -1018,3 +1043,4 @@ <h2>Privacy and Security</h2>
 [Notification interface]: https://www.w3.org/TR/notifications/#notification
 [PaymentRequest interface]: https://www.w3.org/TR/payment-request/#paymentrequest-interface
 [PushManager interface]: https://w3c.github.io/push-api/#idl-def-PushManager
+[requestMIDIAccess method]: http://webaudio.github.io/web-midi-api/#requestMIDIAccess