Bug 1527380 - Re-enable fission test for WebRender. r=rhunt

Differential Revision: https://phabricator.services.mozilla.com/D27699

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

dom/ipc/tests/mochitest.ini

@@ -24,6 +24,6 @@ support-files =
 [test_Preallocated.html]
 skip-if = !e10s || toolkit == 'android' # Bug 1525959
 [test_force_oop_iframe.html]
-skip-if = !e10s || webrender  # oop-iframes trigger a debug assertion in webrender picture caching
+skip-if = !e10s
 support-files =
   file_dummy.html