Added new VideoLayout message that's sent from host to client when
using WebRTC protocol. The message is used to notify the client about
screen configuration. Currently both host and client support only a
single video stream, but in the future the same VideoLayout message
can be used to specify layout of multiple video streams.
When using ICE protocol the message is not sent and instead it's
generated on the client side by ClientVideoDispatcher.

Review URL: https://codereview.chromium.org/1844143002

Cr-Commit-Position: refs/heads/master@{#384440}