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.js

@@ -31,10 +31,15 @@ export type RawPerformanceEntry = {|
   interactionId?: number,
 |};
 
+export type GetPendingEntriesResult = {|
+  entries: $ReadOnlyArray<RawPerformanceEntry>,
+  droppedEntriesCount: number,
+|};
+
 export interface Spec extends TurboModule {
   +startReporting: (entryType: string) => void;
   +stopReporting: (entryType: string) => void;
-  +popPendingEntries: () => $ReadOnlyArray<RawPerformanceEntry>;
+  +popPendingEntries: () => GetPendingEntriesResult;
   +setOnPerformanceEntryCallback: (callback?: () => void) => void;
 }