[Chromoting] Implement new APIs in DesktopCapturer

This change implements GetSourceList() and SelectSource() function in all
DesktopCapturer derived classes.

After this change, and https://codereview.chromium.org/2479553006/,
GetSourceList() and SelectSource() can be changed to pure virtual functions in
WebRTC.

BUG=webrtc:6513

Review-Url: https://codereview.chromium.org/2483483004
Cr-Commit-Position: refs/heads/master@{#430183}
10 files changed