Increase AsyncExtensionSettingsWebUITest.testDeveloperModeA11y timout.

Increasing timeout (again) to see if it helps with the DrMemory test
failures.

NOTRY=true
TBR=finnur

BUG=451299

Review URL: https://codereview.chromium.org/878533002

Cr-Commit-Position: refs/heads/master@{#313054}

chrome/browser/ui/webui/extensions/extension_settings_browsertest.js

@@ -89,7 +89,7 @@ TEST_F('AsyncExtensionSettingsWebUITest', 'testDeveloperModeA11y', function() {
       testDone();
     }
   });
-  ensureTransitionEndEvent(devControl, 2000);
+  ensureTransitionEndEvent(devControl, 4000);
 });
 
 /**