https://cs.chromium.org/chromium/src/third_party/blink/perf_tests/layout/Shapes/MultipleShapes.html

The test does document.write() of html content and then forces layout immediately without waiting for an external stylesheet to load.

The test should wait for the document's load event before running layout.

It's also worth pointing out that this test is not very narrowly focused on layout time; it also includes html parsing time. Ideally, the time measurement would be limited to the PerfTestRunner.forceLayout() call.