Implement and use new DesktopCapturer APIs in WebRTC

This change replaces all GetWindowList / GetScreenList with GetScreenList,
SelectWindow / SelectScreen with SelectSource, and BringSelectedWindowToFront
with FocusOnSelectedSource in WebRTC.

BUG=webrtc:6513

Review-Url: https://codereview.webrtc.org/2479553006
Cr-Original-Commit-Position: refs/heads/master@{#14960}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: fce4905987d650a3119b1071d42166c8d430d4d3
22 files changed