Add command-line param to screenshare_loopback to specify a list of slides

BUG=none

Review-Url: https://codereview.webrtc.org/2814023003
Cr-Original-Commit-Position: refs/heads/master@{#17670}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 8d8185c7743f2713908a1823656493c13fc8ea67
3 files changed