Bug 1557052: Add a bug number to a ini file. r=jib

Differential Revision: https://phabricator.services.mozilla.com/D34394

--HG--
extra : moz-landing-system : lando

testing/web-platform/meta/webrtc/RTCDataChannel-send.html.ini

@@ -1,4 +1,5 @@
 [RTCDataChannel-send.html]
   [Calling send() up to max size should succeed, above max size should fail]
     expected: FAIL
+    bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1278299