Use simulcast for screenshare only in conference mode

Also, don't crash if InitEncode fails for vp8.

BUG=chromium:705505

Review-Url: https://codereview.webrtc.org/2779163002
Cr-Original-Commit-Position: refs/heads/master@{#17452}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: fe627f30cbd3b4e0665237c7f309973cbf15e30b
4 files changed