Bug 1386807 - Part 4: Add hidden WebExtension window to a11y tests to make them pass r=eeejay

This is actually a workaround for the test not ignoring hidden windows. There is
ongoing work in bug 1440106 that will get rid of this issue altogether, but
for now, we need to add hidden windows ourselves.

MozReview-Commit-ID: 2rQoz4KLHv3

--HG--
extra : rebase_source : 265fcd5755339493f320357553407edc33093376

accessible/tests/mochitest/events/docload/test_docload_root.html

@@ -73,6 +73,9 @@
             {
               role: ROLE_CHROME_WINDOW
             },
+            {
+              role: ROLE_CHROME_WINDOW
+            },
             {
               role: ROLE_CHROME_WINDOW
             }

accessible/tests/mochitest/events/docload/test_docload_shutdown.html

@@ -89,6 +89,9 @@
             {
               role: ROLE_CHROME_WINDOW
             },
+            {
+              role: ROLE_CHROME_WINDOW
+            },
             {
               role: ROLE_CHROME_WINDOW
             }