Bug 1638703 - Flush layout explicitly in some a11y non-e10s tests. r=Jamie

These tests insert a <browser> into the DOM via tabbrowser, and
immediately try to get a hold to the underlying document right after.

However the a11y tree is constructed after frame construction, and with
bug 1584935 there's no longer a guarantee that it gets done in time
without these explicit flushes.

Note that I couldn't reproduce these failures locally, so take the
diagnostic with a grain of salt, but I believe it's accurate and that
this is the right fix.

Differential Revision: https://phabricator.services.mozilla.com/D75715

accessible/tests/mochitest/events/test_scroll.xhtml

@@ -44,6 +44,8 @@
         tabBrowser().loadURI(aURL, {
           triggeringPrincipal: Services.scriptSecurityManager.getSystemPrincipal(),
         });
+        // Flush layout, so as to guarantee that the a11y tree is constructed.
+        browserDocument().documentElement.getBoundingClientRect();
       }
 
       this.getID = function loadTab_getID()
@@ -71,6 +73,8 @@
           inBackground: true,
           triggeringPrincipal: Services.scriptSecurityManager.getSystemPrincipal(),
         });
+        // Flush layout, so as to guarantee that the a11y tree is constructed.
+        browserDocument().documentElement.getBoundingClientRect();
       }
 
       this.getID = function loadTabInBackground_getID()

accessible/tests/mochitest/relations/test_tabbrowser.xhtml

@@ -39,6 +39,8 @@
           replace: true,
           triggeringPrincipal: Services.scriptSecurityManager.getSystemPrincipal(),
         });
+        // Flush layout, so as to guarantee that the a11y tree is constructed.
+        browserDocument().documentElement.getBoundingClientRect();
       }
 
       this.finalCheck = function testTabRelations_finalCheck(aEvent)

accessible/tests/mochitest/tree/test_tabbrowser.xhtml

@@ -38,6 +38,8 @@
           replace: true,
           triggeringPrincipal: Services.scriptSecurityManager.getSystemPrincipal(),
         });
+        // Flush layout, so as to guarantee that the a11y tree is constructed.
+        browserDocument().documentElement.getBoundingClientRect();
       }
 
       this.finalCheck = function testTabHierarchy_finalCheck(aEvent)