Add 'contextmenu' as MouseEvent type
Add 'contextmenu' as MouseEvent type

Summary:
It's another `MouseEvent` https://developer.mozilla.org/en-US/docs/Web/Events/contextmenu
Closes https://github.com/facebook/flow/pull/2340

Differential Revision: D3776982

Pulled By: samwgoldman

fbshipit-source-id: 168a289aae23bcf117763ee9b71b0fc9fa4d058c

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