Now RendererImpl only takes DemuxerStreamProvider (minimal interface it needs)
instead of the full Demuxer. Also, this makes other Renderer implementations
easier since they don't necessarily have access to a full Demuxer.

BUG=392236
TEST=No functionality change and all existing tests pass.

Review URL: https://codereview.chromium.org/523283002

Cr-Commit-Position: refs/heads/master@{#294081}