GitLab

This CL begins moving MSE init segment received algorithm implementation
to blink level, so that it could be shared across different
implementations (e.g. between Chrome and Opera). The old init segment
algorithm is in MediaSourceState::OnNewConfigs, and for now it needs to
be kept, since it does other important things that must be done on the
Chromium media pipeline level atm (e.g. creating track buffers and
demuxer streams).

BUG=620881

Review-Url: https://codereview.chromium.org/1678523003
Cr-Commit-Position: refs/heads/master@{#403286}