Bug 1145049 - Fix browser_toolbar_webconsole_errors_count when run independently. r=bgrins

DeveloperToolbar.show now returns a promise.

This code was missed during the refactoring as it wasn't executed
because some previous test wasn't cleaning up correctly and the toolbar was still on.

browser/devtools/shared/test/browser_toolbar_webconsole_errors_count.js

@@ -28,7 +28,7 @@ function test() {
     expectUncaughtException();
 
     if (!DeveloperToolbar.visible) {
-      DeveloperToolbar.show(true, onOpenToolbar);
+      DeveloperToolbar.show(true).then(onOpenToolbar);
     }
     else {
       onOpenToolbar();