Clarify value of entry.name: empty string or "layout-shift"

[Elchi3]

https://wicg.github.io/layout-instability/#report-the-layout-shift says

Set newEntry’s name attribute to "layout-shift".

However, Chromiums return an empty string and this test also asserts "": https://github.com/web-platform-tests/wpt/blob/0c1b9d1622/layout-instability/buffer-layout-shift.html#L25