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

yoga/event/event.h

@@ -15,7 +15,13 @@ namespace facebook {
 namespace yoga {
 
 struct Event {
-  enum Type { NodeAllocation, NodeDeallocation, NodeLayout };
+  enum Type {
+    NodeAllocation,
+    NodeDeallocation,
+    NodeLayout,
+    LayoutPassStart,
+    LayoutPassEnd
+  };
   class Data;
   using Subscriber = void(const YGNode&, Type, Data);