Add DemuxerType enum

This will allow us to remove the rawptr for chunk_demuxer_ in
WebMediaPlayerImpl, and switch behavior based on a new internal field
|demuxer_type_|.

Bug: 1377053
Change-Id: I7854b68cf9639a38796b6e4271441d915007f44e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3969058
Commit-Queue: Ted (Chromium) Meyer <tmathmeyer@chromium.org>
Reviewed-by: Fabrice de Gans <fdegans@chromium.org>
Reviewed-by: Will Cassella <cassew@chromium.org>
Reviewed-by: Matthew Wolenetz <wolenetz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1063625}