I have chosen part of 2435603010 changes to compose this change.
According to the discussion we have made in previous change, this CL contains,
1. Source structure to represent a source of a DesktopCapturer.
2. GetSourceList / SelectSource / FocusOnSelectedSource functions in
DesktopCapturer.
3. ScreenCapturer and WindowCapturer forward corresponding functions to the new
DesktopCapturer APIs.
After this change, We can remove WindowCapturer & ScreenCapturer references from
Chromium, and use the new APIs.
BUG=webrtc:6513
Review-Url: https://codereview.webrtc.org/2452263003
Cr-Commit-Position: refs/heads/master@{#14830}