Fixes bug where QualityScaler would be stuck "way below" QVGA (due to downscale_shift_) even though it would never scale below QVGA. Also fixes issue where samples would be cleared when either staying at max resolution or going below QVGA even though no action happened. BUG= R=glaznev@webrtc.org Review URL: https://codereview.webrtc.org/1971693003 . Cr-Commit-Position: refs/heads/master@{#12691}