Update CLS reporter

[voronianski]

Summarize the feature

Recently Google CLS doc was updated with adjustment in CLS tracking - https://web.dev/cls/#measure-cls-in-javascript. We need to update our reporter too.

Basic example

Taken from the doc above -

      // If the entry occurred less than 1 second after the previous entry and
      // less than 5 seconds after the first entry in the session, include the
      // entry in the current session. Otherwise, start a new session.
      if (sessionValue &&
          entry.startTime - lastSessionEntry.startTime < 1000 &&
          entry.startTime - firstSessionEntry.startTime < 5000) {
        sessionValue += entry.value;
        sessionEntries.push(entry);
      } else {
        sessionValue = entry.value;
        sessionEntries = [entry];
      }

Motivation

Allows more precise tracking.