Summary:
Changelog: [internal]
This is a mechanism that will guess event's React priority based on other events ongoing on the platform.
If an event happens within span of ContinuousStart -> ContinuousEnd and its category is unspecified, we deduce it's React priority to be default. All other events are discrete.
Special case: `onScroll`, which is always treated as "Default".
Reviewed By: JoshuaGross
Differential Revision: D28485060
fbshipit-source-id: d2eae63dbcf03271dfed97128a1590dd165a3ce2