Incorrect function definition of createAnswer and createOffer in lib.dom.d.ts

[lgoenner]

The current definitions of createAnswer and createOffer of RTCPeerConnection don't match the functions defined in the W3C specification.

createAnswer(successCallback?: RTCSessionDescriptionCallback, failureCallback?: RTCPeerConnectionErrorCallback): Promise<RTCSessionDescription>;
createOffer(successCallback?: RTCSessionDescriptionCallback, failureCallback?: RTCPeerConnectionErrorCallback, options?: RTCOfferOptions): Promise<RTCSessionDescription>;

W3C interface specification:

Promise<RTCSessionDescriptionInit> createOffer(optional RTCOfferOptions options);
Promise<RTCSessionDescriptionInit> createAnswer(optional RTCAnswerOptions options);

W3C legacy interface extensions:

Promise<void> createOffer(RTCSessionDescriptionCallback successCallback, RTCPeerConnectionErrorCallback failureCallback, optional RTCOfferOptions options);
Promise<void> createAnswer(RTCSessionDescriptionCallback successCallback, RTCPeerConnectionErrorCallback failureCallback);

[mhegazy]

PRs welcomed. You can find more information about contributing lib.d.ts fixes at https://github.com/Microsoft/TypeScript/blob/master/CONTRIBUTING.md#contributing-libdts-fixes.