Permits overriding the source-default is_screencast option to be able to
treat screencast sources as fluid video, preserving motion at the loss
of individual frame quality (or vice versa).
BUG=chromium:653531
R=deadbeef@webrtc.org
Review-Url: https://codereview.webrtc.org/2579993003
Cr-Commit-Position: refs/heads/master@{#15659}