Ensure CLS is only reported if page was visible

[philipwalton]

Currently, in CrUX, CLS will not be reported for a page if it does not report any FCP value. This was done to filter out pages reporting 0 when they were loaded in the background and never focused.

However, matching the CrUX behavior here is problematic for two reasons:

• CrUX is actually under-reporting CLS because pages that were loaded in the background and later focused can still have layout shifts that affect the user experience. Yet these pages will not report FCP, and thus those layout shifts are ignored. (CrUX plans to fix this in the future.)
• Many users of this library want to capture CLS, but they may not be interested in capturing FCP. Yet if the getCLS() function is coupled to the FCP logic, the final bundle size for those users will be larger than it needs to be for users only interested in the Core Web Vitals.

To address both of these points—and anticipate the changes CrUX plans to make—this PR updates the logic to report CLS if the page was ever visible at any point in its lifecycle.