This avoids the situation where an encoder, not supporting certain
screen content settings, is created for a config where screencast is
off, and later ReconfigureEncoder() is called updating the configuration
but not the encoder instance, causing an inconsistency in the encoder's
InitEncode() call.
TBR=pthatcher@webrtc.org
BUG=webrtc:4172
Review-Url: https://codereview.webrtc.org/2710493008
Cr-Commit-Position: refs/heads/master@{#16921}
f24a06406a