Limit perf entry buffer size and send back 'droppedEntriesCount' to PerformanceObserver callback

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

Libraries/WebPerformance/NativePerformanceObserver.cpp

@@ -40,7 +40,7 @@ void NativePerformanceObserver::stopReporting(
       stringToPerformanceEntryType(entryType));
 }
 
-std::vector<RawPerformanceEntry> NativePerformanceObserver::popPendingEntries(
+GetPendingEntriesResult NativePerformanceObserver::popPendingEntries(
     jsi::Runtime &rt) {
   return PerformanceEntryReporter::getInstance().popPendingEntries();
 }