RTCPeerConnectionFactory.createPeerConnection did not check the return
value of the native createPeerConnection() - so when the native PC
fails to be created, it could end up attempting to use a null pointer.
The change makes it return nil when the creation fails. The application
can then detect and respond to the failure.
Review-Url: https://codereview.webrtc.org/2240633004
Cr-Commit-Position: refs/heads/master@{#13732}
588783adcd