Add requestIdleCallback/cancelIdleCallback defs
Add requestIdleCallback/cancelIdleCallback defs

Summary:
See https://developer.mozilla.org/en-US/docs/Web/API/Window/requestIdleCallback
Closes https://github.com/facebook/flow/pull/2800

Differential Revision: D4174175

Pulled By: mroch

fbshipit-source-id: 39223bed4d6a28a394a7df25d0aed0fa1945f57c

gabro/flow,nmote/flow,nmote/flow,claudiopro/flow,TiddoLangerak/flow,samwgoldman/flow,samwgoldman/flow,facebook/flow,jamesgpearce/flow,JonathanUsername/flow,facebook/flow,samwgoldman/flow,claudiopro/flow,MichaelDeBoey/flow,facebook/flow,gabelevi/flow,AgentME/flow,fletcherw/flow,samwgoldman/flow,ForbesLindesay/flow,fletcherw/flow,nmote/flow,nmote/flow,ForbesLindesay/flow,gabro/flow,fletcherw/flow,facebook/flow,AgentME/flow,ForbesLindesay/flow,popham/flow,JonathanUsername/flow,ForbesLindesay/flow,MichaelDeBoey/flow,MichaelDeBoey/flow,ForbesLindesay/flow,TiddoLangerak/flow,facebook/flow,claudiopro/flow,claudiopro/flow,mroch/flow,samwgoldman/flow,TiddoLangerak/flow,popham/flow,nmote/flow,ylu1317/flow,JonathanUsername/flow,ylu1317/flow,samwgoldman/flow,gabelevi/flow,mroch/flow,mroch/flow,JonathanUsername/flow,gabro/flow,mroch/flow,nmote/flow,gabro/flow,fletcherw/flow,mroch/flow,jamesgpearce/flow,popham/flow,jamesgpearce/flow,AgentME/flow,popham/flow,mroch/flow,MichaelDeBoey/flow,fletcherw/flow,mroch/flow,MichaelDeBoey/flow,claudiopro/flow,popham/flow,jamesgpearce/flow,facebook/flow,gabelevi/flow,gabelevi/flow,ylu1317/flow,TiddoLangerak/flow,ForbesLindesay/flow,fletcherw/flow,samwgoldman/flow,ylu1317/flow,JonathanUsername/flow,gabro/flow,popham/flow,AgentME/flow,gabelevi/flow,jamesgpearce/flow,jamesgpearce/flow,popham/flow,JonathanUsername/flow,nmote/flow,AgentME/flow,jamesgpearce/flow,JonathanUsername/flow,facebook/flow,TiddoLangerak/flow,gabro/flow,gabelevi/flow,ylu1317/flow,gabelevi/flow