Bug 1824791: Mark WebGPU data as reported when generating about:support. r=Gijs

The `about:support` page formatter for graphics information takes a JavaScript object (the `graphics` property of the snapshot returned by `Troubleshoot.snapshot`), and builds HTML covering each property of the object. It deletes properties as it goes, and at the end, any remaining properties get dumped into a "diagnostics" section.

Fix the WebGPU adapter reporting code to delete the properties it reports, so that they don't end up generating stray rows in the diagnostics section.

Differential Revision: https://phabricator.services.mozilla.com/D173751

toolkit/content/aboutSupport.js

@@ -747,6 +747,7 @@ var snapshotFormatters = {
       if (obj) {
         const str = JSON.stringify(obj, null, "  ");
         await addRow("features", feature, [new Text(str)]);
+        delete data[feature];
       }
     }