[PR] Fix getContext return type
[PR] Fix getContext return type

Summary:
https://developer.mozilla.org/en-US/docs/Web/API/HTMLCanvasElement/getContext

`canvas.getContext('2d')` is supported in all browsers, so we shouldn't have to null-check it.
Closes https://github.com/facebook/flow/pull/4126

Differential Revision: D5246294

Pulled By: mroch

fbshipit-source-id: f21f86a461a5f5bcd3d91b52344a047c2d565246

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