Add backgroundSegmentationMask to VideoFrameMetadata registry

Author: eehakkin

video_frame_metadata_registry.src.html

@@ -77,6 +77,10 @@
     <td>backgroundBlur</td>
     <td>[Background blur effect status](https://w3c.github.io/mediacapture-extensions/#background-blur-effect-status)</td>
   </tr>
+  <tr>
+    <td>backgroundSegmentationMask</td>
+    <td>[Background segmentation mask](https://github.com/w3c/mediacapture-extensions/pull/142)</td>
+  </tr>
 </table>