Publish events for layout pass

Summary:
Adds `LayoutPassStart` and `LayoutPassEnd` events.

The existing `NodeLayout` event in isolation is not as useful as it could be. Having events that mark start and end of a layout pass are a useful addition.

Differential Revision: D15305467

fbshipit-source-id: 14af6f65e698fb1e3112eb2ffd87a74d31df4840

tests/EventsTest.cpp

@@ -127,6 +127,10 @@ void EventTest::listen(const YGNode& node, Event::Type type, Event::Data data) {
     case Event::NodeLayout:
       events.push_back(createArgs<Event::NodeLayout>(node, data));
       break;
+    case Event::LayoutPassStart:
+      break;
+    case Event::LayoutPassEnd:
+      break;
   }
 }