[PR] Fix IntersectionObserver constructor definition
[PR] Fix IntersectionObserver constructor definition

Summary:
The options argument in IntersectionObserver's constructor should be optional:
https://w3c.github.io/IntersectionObserver/#intersection-observer-interface
Pull Request resolved: https://github.com/facebook/flow/pull/4664

Reviewed By: dsainati1

Differential Revision: D13866173

Pulled By: nmote

fbshipit-source-id: c3836c7be6e6e313b5b2b3723ccfb01166063dd6

nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow