allow MediaRecorder be created with only mimeType
allow MediaRecorder be created with only mimeType

Summary: currently when supplying MediaRecorderOptions to MediaRecorder constructor, it expects MediaRecorderOptions have several required parameters which are actually optional.

Reviewed By: mroch

Differential Revision: D27982568

fbshipit-source-id: 7f81f5924a6bc57418652a86640e20a450021944

facebook/flow,mroch/flow,mroch/flow,facebook/flow,mroch/flow,facebook/flow,mroch/flow,mroch/flow,mroch/flow,facebook/flow,facebook/flow,facebook/flow,facebook/flow,mroch/flow