Bug 1634337 - mark offscreen-canvas tests as backlog (tier-2). r=jgraham

mark offscreen-canvas tests as backlog (tier-2).

Differential Revision: https://phabricator.services.mozilla.com/D73305

testing/web-platform/meta/imagebitmap-renderingcontext/toBlob-origin-clean-offscreen.sub.html.ini

@@ -1,3 +1,5 @@
+implementation-status: backlog
+bug: 1390089
 [toBlob-origin-clean-offscreen.sub.html]
   [Test that call convertToBlob on a tainted OffscreenCanvas throws exception]
     expected: FAIL

testing/web-platform/meta/offscreen-canvas/__dir__.ini

@@ -0,0 +1,2 @@
+implementation-status: backlog
+bug: 1390089