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.cpp

@@ -17,6 +17,10 @@ static PerformanceEntryType stringToPerformanceEntryType(
     return PerformanceEntryType::MARK;
   } else if (entryType == "measure") {
     return PerformanceEntryType::MEASURE;
+  } else if (entryType == "event") {
+    return PerformanceEntryType::EVENT;
+  } else if (entryType == "first-input") {
+    return PerformanceEntryType::FIRST_INPUT;
   } else {
     return PerformanceEntryType::UNDEFINED;
   }