support buffered PerformanceObserver

[twmillett]

It looks like Chromium 77 now supports the buffered flag.

Would be awesome if the tti-polyfill had a config option to take advantage of that, so the workaround mentioned in the readme is no longer necessary.

[filmic]

Buffered flag does not support the 'longtask' entry type (checked in Chrome 77.0.3865.90).

[tdresser]

FYI - Chromium may support the buffered flag for longtasks eventually, we just need to measure the performance implications of storing long task entries for all pages, including those that never read them.

[jdlm-stripe]

Looks like buffered with longtask was added in Chrome 81. I'd also like to remove the workaround.