This CL adds a fallback for the case when no optimized scale factor produces a low enough resolution for what was requested. It also ensures that all resolutions provided by the video adapter are divisible by four. This is required by some hardware implementations. BUG=webrtc:6837 Review-Url: https://codereview.webrtc.org/2555483005 Cr-Commit-Position: refs/heads/master@{#15469}