cleanup: Only omit braces for single-line blocks

Braces can be avoided when a block consists of a single statement,
but readability suffers when the statement spans more than a single
line.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/805

js/perf/core.js

@@ -94,11 +94,12 @@ function *run() {
             let config = WINDOW_CONFIGS[i / 2];
             yield Scripting.destroyTestWindows();
 
-            for (let k = 0; k < config.count; k++)
+            for (let k = 0; k < config.count; k++) {
                 yield Scripting.createTestWindow({ width: config.width,
                                                    height: config.height,
                                                    alpha: config.alpha,
                                                    maximized: config.maximized });
+            }
 
             yield Scripting.waitTestWindows();
             yield Scripting.sleep(1000);