API to report events

Summary:
Extends the WebPerformance API with ability to report events, [according to the standard](https://www.w3.org/TR/event-timing/#sec-performance-event-timing).

This is an API-only change, the actual reporting comes in a separate diff, to simplify reviewing.

Changelog: [Internal]

Reviewed By: christophpurrer

Differential Revision: D42097695

fbshipit-source-id: d8b468ffed50c1c3d889151df5e8ca644d6e1a68

Libraries/WebPerformance/NativePerformanceObserver.h

@@ -26,7 +26,7 @@ using RawPerformanceEntry = NativePerformanceObserverCxxBaseRawPerformanceEntry<
     // For "event" entries only:
     std::optional<double>,
     std::optional<double>,
-    std::optional<double>>;
+    std::optional<uint32_t>>;
 
 template <>
 struct Bridging<RawPerformanceEntry>
@@ -37,7 +37,7 @@ struct Bridging<RawPerformanceEntry>
           double,
           std::optional<double>,
           std::optional<double>,
-          std::optional<double>> {};
+          std::optional<uint32_t>> {};
 
 using GetPendingEntriesResult =
     NativePerformanceObserverCxxBaseGetPendingEntriesResult<