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}