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