This is instead derived from pattern_idx inside the frame config.
This also removes active_layer_ use from
ScreenshareLayers::PopulateCodecSpecific and instead ties the layer to
TemporalLayers::FrameConfig.
BUG=chromium:702017, webrtc:7349
R=sprang@webrtc.org
Review-Url: https://codereview.webrtc.org/2860063002
Cr-Commit-Position: refs/heads/master@{#18017}