Previously raw pointers were used for owned DesktopFrame instances.
Updated all screen and window capturer implementations to use
std::unique_ptr<>.
Also includes some other cleanups in the capturers:
- s/NULL/nullptr
- moved default initializers to class definition.
BUG=webrtc:5950
Review-Url: https://codereview.webrtc.org/1988783003
Cr-Commit-Position: refs/heads/master@{#13058}