SurfaceLayerForMediaStreams: cleanup launched feature.

This CL removes the SurfaceLayerForMediaStreams base::Feature which is
fully launched. SurfaceLayer video continue to be conditionally enabled
only on the Android platform.

The CL achieves a small cleanup in the MediaFactory functions to
simplify logic.

The CL also removes RenderThreadImpl::SetVideoFrameCompositorTaskRunner
since SurfaceLayerMode::kOnDemand isn't present anymore.

Bug: 1157671, 901513
Change-Id: I0f525185b331fd128bbc8096e9604d8c25c38615
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3625969


Commit-Queue: Markus Handell <handellm@google.com>
Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Reviewed-by: Maggie Chen <magchen@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#999805}