Summary:
Prevents scenarios when internal performance buffer may grow indefinitely (e.g. due to a broken logging), communicating back to `PerformanceObserver` the corresponding amount of dropped entries, `droppedEntriesCount`, [according to the standard](https://w3c.github.io/performance-timeline/#dom-performanceobservercallbackoptions-droppedentriescount).
NOTE: The backwards compatibility check is failing, which is an orthogonal issue. I am looking into it and won't land this one before it is sorted.
Changelog: [Internal]
Reviewed By: christophpurrer
Differential Revision: D42008409
fbshipit-source-id: 40d30e44d39e643bfb58a6254572823cb2b3b8df