Fabric: Making EventBeat::beat protected member

Summary:
The documentation for the method says that it must be called from a subclass, but the methods wasn't marked as protected.

Calling `beat` directly will bypass custom logic implemented in subclasses (because it's not a virtual function (by-design)). That's practically what happened before and what the previous diffs in the stack fix.

Changelog: [Internal]

Reviewed By: JoshuaGross

Differential Revision: D18339594

fbshipit-source-id: 3d8c53b69e1b72d447714dd91c9392bdfb05b9ae

ReactCommon/fabric/core/events/EventBeat.h

@@ -75,13 +75,13 @@ class EventBeat {
    */
   void setBeatCallback(const BeatCallback &beatCallback);
 
+ protected:
   /*
    * Should be used by sublasses to send a beat.
    * Receiver might ignore the call if a beat was not requested.
    */
   void beat(jsi::Runtime &runtime) const;
 
- protected:
   BeatCallback beatCallback_;
   SharedOwnerBox ownerBox_;
   mutable std::atomic<bool> isRequested_{false};